surface-actor: Unref shaped texture on dispose

The MetaShapedTexture created by MetaSurfaceActor used to
be a ClutterActor, which means destruction was taken care
by Clutter.

Now that it's a plain GObject, we need to manually clean it
up.

Cleanup the shaped texture on disposal.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/753

src/compositor/meta-surface-actor.c

@@ -202,6 +202,7 @@ meta_surface_actor_dispose (GObject *object)
     meta_surface_actor_get_instance_private (self);
 
   g_clear_pointer (&priv->input_region, cairo_region_destroy);
+  g_clear_object (&priv->texture);
 
   set_unobscured_region (self, NULL);
   set_clip_region (self, NULL);