offscreen: Use the allocation box to get the size of the actor in paint
offscreen: Use the allocation box to get the size of the actor in paint

When painting, we were using clutter_actor_get_size to get the actor's
size when painting the accumulation buffer. This doesn't always do what
you expect, and instead, we want to use the allocation box.

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