meson: list both AVIF and HEIC mimetype support if the plug-in is built.

Even though, some format support might be missing, since codecs can be added at
runtime, it can always change later without having to touch GIMP. Yet we won't
be able to edit the desktop file when this happens.

See new comments in #9080.

meson.build

@@ -795,13 +795,6 @@ if have_heif
   dependencies: [ libheif ],
   name: 'export HEIC').returncode() == 0
 
-  if can_import_heic
-    MIMEtypes += [
-      'image/heif',
-      'image/heic',
-    ]
-  endif
-
   can_import_avif = cc.run('''
   #include <libheif/heif.h>
   int main() {
@@ -844,16 +837,21 @@ if have_heif
   dependencies: [ libheif ],
   name: 'export AVIF').returncode() == 0
 
-  if can_import_avif
-    MIMEtypes += [
-      'image/avif'
-    ]
-  endif
-
   if not can_import_heic and not can_import_avif
     have_heif = false
   endif
 
+  if have_heif
+    # Listing support for both HEIC and AVIF if we build with HEIF support,
+    # because codecs can be added at any time later and we won't be able to edit
+    # the desktop file once it's installed. See discussion in #9080.
+    MIMEtypes += [
+      'image/heif',
+      'image/heic',
+      'image/avif'
+    ]
+  endif
+
   if have_heif and (libheif.version().version_compare('==1.5.0') or libheif.version().version_compare('==1.5.1'))
     libheif_warning='''