stage: Destroy all children when we dispose

Destroying an actor is supposed to destroy all of its children, so
it makes sense to reason that destroying the stage should destroy all
of its children, too.

Unfortunately, it seems that the stage removed all of its children
without destroying them before chaining up to what would destroy all
of its children, for whatever reason. Change this to a destroy so
resources get cleaned up.

clutter/clutter-stage.c

@@ -1899,7 +1899,7 @@ clutter_stage_dispose (GObject *object)
       priv->impl = NULL;
     }
 
-  clutter_actor_remove_all_children (CLUTTER_ACTOR (object));
+  clutter_actor_destroy_all_children (CLUTTER_ACTOR (object));
 
   g_list_free_full (priv->pending_queue_redraws,
                     (GDestroyNotify) free_queue_redraw_entry);