plug-ins: new JPEG export option to value file size over encoding speed.

This uses MozJPEG from Mozilla and is dependent on this patch to be merged:
https://github.com/mozilla/mozjpeg/pull/383
This is an optional option (standard libjpeg API, e.g. with jpeg-turbo, is still
used when MozJPEG is not detected at build time).

When using MozJPEG, we have access to both the standard algorithm used by
libjpeg-turbo, since MozJPEG is a patched version of libjpeg-turbo, and their
own encoding algorithm. We can switch from one to another with a single call.

Here is what the maintainer says about MozJPEG goal:

> The point of MozJPEG is to improve quality/filesize ratio. It's a win-win: you
> get better quality for the same file size, or better file size for the same
> quality, or both. There is no downside in either quality or file size. MozJPEG
> tunes for these two aspects over speed. libjpeg-turbo's maintainer values speed
> over the other two variables.
>
> MozJPEG has a few techniques. Improved splitting of progressive scans gives
> smaller file size while being 100% visually identical with libjpeg-turbo.
>
> But MozJPEG also has trellis quantization and tuned quantization tables that
> give better visual quality, but on a microscopic scale they make different
> choices than libjpeg-turbo, so some pixels differ. The differences are
> relatively small and predictable, so there's no risk of unexpectedly ruining
> an image (especially that on average, you get better quality).

Cf. https://github.com/mozilla/mozjpeg/issues/382#issuecomment-730544796

Note that after several testing, I could indeed confirm that it seems to always
produce smaller files (as far as my testing went) for similarly looking quality,
but the speed cost can actually be quite important: on my computer, for some
random files where encoding would take 0.7 second, it took 3.5 secs with
mozjpeg; for much bigger file (~25MiB) where export with jpeg-turbo takes about
3.9 secs, it takes 30+ seconds with MozJPEG which is a huge difference and can
be very frustrating.
For small files only, this is less of a problem (I still timed an important
difference, but from 0.05 to 0.15 secs is actually bearable).

This is why this cannot be an option checked by default.

About naming: I hesitated to call it "Export for Web" because it's clearly one
of the big use cases (optimizing file size for images on websites), but I just
decided to go with a much more explicit name (even though it may resonate less
that the basic "Export for Web" which everyone asks for).

meson.build

@@ -667,7 +667,13 @@ libtiff = dependency('libtiff-4', version: '>=' + libtiff_minver)
 MIMEtypes += 'image/tiff'
 
 
-libjpeg = dependency('libjpeg')
+mozjpeg = dependency('mozjpeg', required: get_option('mozjpeg'))
+if mozjpeg.found()
+  libjpeg = mozjpeg
+else
+  libjpeg = dependency('libjpeg')
+endif
+conf.set('HAVE_MOZJPEG', mozjpeg.found())
 MIMEtypes += 'image/jpeg'
 
 
@@ -1952,6 +1958,7 @@ final_message = [
 '''  Detailed backtraces:       @0@'''.format(detailed_backtraces),
 '''  Binary symlinks:           @0@'''.format(enable_default_bin),
 '''  OpenMP:                    @0@'''.format(have_openmp),
+'''  MozJPEG:                   @0@'''.format(mozjpeg.found()),
 '',
 '''Optional Plug-Ins:''',
 '''  Ascii Art:           @0@'''.format(libaa.found()),

meson_options.txt

@@ -44,6 +44,7 @@ option('ilbm',              type: 'feature', value: 'auto', description: 'Amiga
 option('jpeg2000',          type: 'feature', value: 'auto', description: 'Jpeg-2000 support')
 option('jpeg-xl',           type: 'feature', value: 'auto', description: 'JPEG XL support')
 option('mng',               type: 'feature', value: 'auto', description: 'Mng support')
+option('mozjpeg',           type: 'feature', value: 'auto', description: 'Build JPEG support with mozjpeg specifically')
 option('openexr',           type: 'feature', value: 'auto', description: 'Openexr support')
 option('openmp',            type: 'feature', value: 'auto', description: 'OpenMP support')
 option('print',             type: 'boolean', value: true,   description: 'Print support')

plug-ins/file-jpeg/jpeg-save.c

@@ -198,6 +198,7 @@ save_image (GFile                *file,
             GimpImage            *image,
             GimpDrawable         *drawable,
             GimpImage            *orig_image,
+            GimpRunMode           run_mode,
             gboolean              preview,
             GError              **error)
 {
@@ -236,6 +237,7 @@ save_image (GFile                *file,
   gint             orig_num_quant_tables = -1;
   gboolean         use_arithmetic_coding = FALSE;
   gboolean         use_restart           = FALSE;
+  gboolean         mozjpeg               = FALSE;
   gchar           *comment;
 
   g_object_get (config,
@@ -248,6 +250,7 @@ save_image (GFile                *file,
                 "baseline",                  &baseline,
                 "restart",                   &restart,
                 "dct",                       &dct,
+                "use-mozjpeg",               &mozjpeg,
 
                 /* Original quality settings. */
                 "use-original-quality",      &use_orig_quality,
@@ -262,6 +265,16 @@ save_image (GFile                *file,
 
                 NULL);
 
+  if (run_mode == GIMP_RUN_NONINTERACTIVE && mozjpeg)
+    {
+#ifndef HAVE_MOZJPEG
+      g_set_error_literal (error, GIMP_PLUG_IN_ERROR, 0,
+                           _("GIMP was not compiled with MozJPEG. "
+                             "The argument 'use-mozjpeg' cannot be set to TRUE."));
+      return FALSE;
+#endif
+    }
+
   quality = (gint) (dquality * 100.0 + 0.5);
 
   drawable_type = gimp_drawable_type (drawable);
@@ -489,6 +502,13 @@ save_image (GFile                *file,
                               drawable_type == GIMP_RGBA_IMAGE)
         ? JCS_RGB : JCS_GRAYSCALE;
     }
+
+#ifdef HAVE_MOZJPEG
+  if (! mozjpeg)
+    /* Disable mozjpeg code path (upstream jpeg-turbo normal algorithm). */
+    jpeg_c_set_int_param (&cinfo, JINT_COMPRESS_PROFILE, JCP_FASTEST);
+#endif
+
   /* Now use the library's routine to set default compression parameters.
    * (You must set at least cinfo.in_color_space before calling this,
    * since the defaults depend on the source color space.)
@@ -795,6 +815,7 @@ make_preview (GimpProcedureConfig *config)
                   preview_image,
                   drawable_global,
                   orig_image_global,
+                  GIMP_RUN_NONINTERACTIVE,
                   TRUE, NULL);
 
       g_object_unref (file);
@@ -995,6 +1016,9 @@ save_dialog (GimpProcedure       *procedure,
                                   "restart-frame",
                                   "sub-sampling",
                                   "dct",
+#ifdef HAVE_MOZJPEG
+                                  "use-mozjpeg",
+#endif
                                   NULL);
   gimp_procedure_dialog_fill_frame (GIMP_PROCEDURE_DIALOG (dialog),
                                     "advanced-frame", "advanced-title", FALSE,

plug-ins/file-jpeg/jpeg-save.h

@@ -24,10 +24,11 @@ extern GimpDrawable    *drawable_global;
 
 
 gboolean    save_image         (GFile                *file,
-                                GimpProcedureConfig *config,
+                                GimpProcedureConfig  *config,
                                 GimpImage            *image,
                                 GimpDrawable         *drawable,
                                 GimpImage            *orig_image,
+                                GimpRunMode           run_mode,
                                 gboolean              preview,
                                 GError              **error);
 gboolean    save_dialog        (GimpProcedure        *procedure,

plug-ins/file-jpeg/jpeg.c

@@ -301,6 +301,11 @@ jpeg_create_procedure (GimpPlugIn  *plug_in,
                                  _("Use restart mar_kers"),
                                  NULL, FALSE,
                                  G_PARAM_READWRITE);
+      GIMP_PROC_AUX_ARG_BOOLEAN (procedure, "use-mozjpeg",
+                                 _("Value file size over encoding speed (using MozJPEG)"),
+                                 _("Use MozJPEG optimizations for better quality/filesize ratio, with slower encoding."),
+                                 FALSE,
+                                 G_PARAM_READWRITE);
 
       gimp_save_procedure_set_support_exif      (GIMP_SAVE_PROCEDURE (procedure), TRUE);
       gimp_save_procedure_set_support_iptc      (GIMP_SAVE_PROCEDURE (procedure), TRUE);
@@ -597,8 +602,8 @@ jpeg_save (GimpProcedure        *procedure,
   if (status == GIMP_PDB_SUCCESS)
     {
       if (! save_image (file, config,
-                        image, drawables[0], orig_image, FALSE,
-                        &error))
+                        image, drawables[0], orig_image,
+                        run_mode, FALSE, &error))
         {
           status = GIMP_PDB_EXECUTION_ERROR;
         }