offscreen: Fix calling queue_relayout during dispose
offscreen: Fix calling queue_relayout during dispose

Calling clutter_actor_queue_relayout on an actor during its dispose cycle
can cause a segfault.

jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx