actor: Add private getter for the active framebuffer

Instead of asking every internal user to get the stage and get the
active framebuffer from it, we can wrap it up ourselves, and do some
sanity checks as well.

clutter/clutter-actor-private.h

@@ -320,6 +320,8 @@ void                            _clutter_actor_queue_redraw_on_clones
 void                            _clutter_actor_queue_relayout_on_clones                 (ClutterActor *actor);
 void                            _clutter_actor_queue_only_relayout                      (ClutterActor *actor);
 
+CoglFramebuffer *               _clutter_actor_get_active_framebuffer                   (ClutterActor *actor);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_ACTOR_PRIVATE_H__ */