Issue #6165: crash when the system icon theme does not contain image-missing icon.

The bug is in fact in GTK. "image-missing" is supposed to never missing since
GTK embeds an icon as GResource. But it turns out that the code is buggy. So if
an icon is missing from GIMP theme, then system theme, then "icon-missing" icon
as well, GIMP crashes.

See: https://gitlab.gnome.org/GNOME/gtk/-/issues/3526

As a workaround for now, let's at least add our own "image-missing", at least in
our main themes (Color and Symbolic, not Legacy). This will make GIMP not
crashing at least by default when another icon is missing.
Of course a crash could still happen if one use Legacy or a third-party theme
and if their system theme doesn't contain "image-missing" as well, until GTK is
fixed.

icons/Color/meson.build

@@ -36,6 +36,13 @@ foreach install : install_icons
   endforeach
 
   install_data(icons, install_dir: gimpiconsdir / theme / outdir / 'apps')
+
+  wilber_eek = '@0@/gimp-wilber-eek@1@'.format(indir, suffix)
+  if wilber_eek in icons
+    install_data(wilber_eek,
+                 rename: 'image-missing@0@'.format(suffix),
+                 install_dir: gimpiconsdir / theme / outdir / 'apps')
+  endif
 endforeach
 
 icons_core = [ '64/dialog-question.png', ]

icons/Symbolic/meson.build

@@ -41,6 +41,18 @@ foreach install : install_icons
   endforeach
 
   install_data(icons, install_dir: gimpiconsdir / theme / outdir / 'apps')
+
+  wilber_eek = '@0@/gimp-wilber-eek@1@'.format(indir, suffix)
+  if wilber_eek in icons
+    # Note: since this is just a copy of gimp-wilber-week, we could just do a
+    # symlink with install_symlink(), but this is only available in meson 0.61.0
+    # and looking at its code, it would apparently fail if symlinks are not
+    # available in the target OS (instead of silently and intelligently falling
+    # back to doing a copy).
+    install_data(wilber_eek,
+                 rename: 'image-missing@0@'.format(suffix),
+                 install_dir: gimpiconsdir / theme / outdir / 'apps')
+  endif
 endforeach
 
 icons_core = [ '64/gimp-question.png', ]