texture-cache: Use content size for returned images

Since commit deec0bf255, the texture cache is based on ClutterImage
rather than ClutterTexture. As ClutterImage (like all ClutterContent)
is only concerned with painting, it doesn't influence the size of the
actor it is added to at all, and the returned actor will now stay at
size 0x0 after the image has been loaded.

Set up the actor to follow the content's size instead, to get closer
to the previous behavior.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/402

src/st/st-texture-cache.c

@@ -71,6 +71,7 @@ create_invisible_actor (void)
 {
   return g_object_new (CLUTTER_TYPE_ACTOR,
                        "opacity", 0,
+                       "request-mode", CLUTTER_REQUEST_CONTENT_SIZE,
                        NULL);
 }