window-actor: Always chain up to parent's dispose vfunc

Our early out shouldn't mean we early out from the parents dispose
function.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/659

src/compositor/meta-window-actor.c

@@ -457,7 +457,10 @@ meta_window_actor_dispose (GObject *object)
   MetaCompositor *compositor = priv->compositor;
 
   if (priv->disposed)
+    {
+      G_OBJECT_CLASS (meta_window_actor_parent_class)->dispose (object);
       return;
+    }
 
   priv->disposed = TRUE;