shaped-texture: Remove silly ClutterPaintVolume code with slightly less silly ClutterActorBox code
window-actor: Base the paint volume on our children, not our shape

This was wrong for subsurfaces that extend beyond the parent's shape,
since the paint volume would be wrong in this case. Instead of using the
shape region which can be out of date and wrong, just use the union of
our children's volumes, which is a lot easier to manage.