When unparenting an actor, remove queued redraws for all descendants

When unparenting an actor, remove queued redraws for all descendants

We need to make sure that redraws queued for actors on a stage are for
actors actually in the stage. So in clutter_actor_unparent() descend
through the children and remove redraws. Just removing the actor itself
isn't good enough since an entire hierarchy can be removed from the
stage without breaking it up into individual actors.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2359

This is based on an original patch from Owen Taylor who debugged the
root cause of this bug; thanks.