texture-cache: Use image-missing image when no other icon was loaded

If a given icon is not available for neither the current theme nor any
of the fallback options (default theme -"Adwaita", "gnome"...), the
call to gtk_icon_theme_lookup_by_gicon() will return a NULL value, which
returned by the Shell Toolkit as is, causing trouble in the Shell's JS
code when calling shell_app_create_icon_texture() to create an icon.

To at least mitigate the chances of this having this issue happening, we
should at least try to load the standad 'image-missing' icon from the
Icon Naming Specification spec when we receive a NULL here, so that
at least we try to show something to the user, even if it's ugly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1121

src/st/st-texture-cache.c

@@ -34,6 +34,8 @@
 #define CACHE_PREFIX_FILE "file:"
 #define CACHE_PREFIX_FILE_FOR_CAIRO "file-for-cairo:"
 
+#define IMAGE_MISSING_ICON_NAME "image-missing"
+
 struct _StTextureCachePrivate
 {
   GtkIconTheme *icon_theme;
@@ -993,8 +995,16 @@ st_texture_cache_load_gicon (StTextureCache    *cache,
   info = gtk_icon_theme_lookup_by_gicon_for_scale (theme, icon,
                                                    size, scale,
                                                    lookup_flags);
+  if (info == NULL)
+    {
+      /* Do not give up without even trying to pick the image-missing fallback icon. */
+      info = gtk_icon_theme_lookup_icon_for_scale (theme,
+                                                   IMAGE_MISSING_ICON_NAME,
+                                                   size, scale,
+                                                   GTK_ICON_LOOKUP_USE_BUILTIN);
       if (info == NULL)
         return NULL;
+    }
 
   gicon_string = g_icon_to_string (icon);
   /* A return value of NULL indicates that the icon can not be serialized,