app, libgimp: Fix warning that fcinit is required since fontconfig 2.17

A recent change in Fontconfig 2.17 is that `FcInit()` became mandatory
for proper initialization. Not having that generates a warning in the
terminal:
`Fontconfig warning: using without calling FcInit()`

We fix this in the same way as GTK3 does: by calling
`pango_cairo_font_map_get_default ()`
See https://gitlab.gnome.org/GNOME/gtk/-/issues/7639

We need to add this in two places, in app for GIMP itself, and in
libgimp for plug-ins.
This adds a dependency on pangocairo for libgimp.

app/main.c

@@ -53,6 +53,8 @@
 #include <gdk-pixbuf/gdk-pixbuf.h>
 #endif
 
+#include <pango/pangocairo.h>
+
 #include <babl/babl.h>
 
 #include "libgimpbase/gimpbase.h"
@@ -644,6 +646,9 @@ main (int    argc,
     g_free (bin_dir);
   }
 
+  /* Trigger fontconfig initialization early */
+  pango_cairo_font_map_get_default ();
+
 #ifndef _WIN64
   {
     typedef BOOL (WINAPI *t_SetProcessDEPPolicy) (DWORD dwFlags);

libgimp/gimp.c

@@ -82,6 +82,8 @@
 
 #include <locale.h>
 
+#include <pango/pangocairo.h>
+
 #include "gimp.h"
 
 #include "libgimpbase/gimpbase-private.h"
@@ -273,6 +275,9 @@ gimp_main (GType  plug_in_type,
   }
 #endif /* HAVE_EXCHNDL */
 
+  /* Trigger fontconfig initialization early */
+  pango_cairo_font_map_get_default ();
+
 #ifndef _WIN64
   {
     typedef BOOL (WINAPI *t_SetProcessDEPPolicy) (DWORD dwFlags);

libgimp/meson.build

@@ -364,6 +364,7 @@ libgimp = library('gimp-'+ gimp_api_version,
     drmingw,
     math,
     pango,
+    pangocairo,
     pangoft2,
   ],
   sources: [