clutter/actor: Dummy set allocation box if transition created

The transition may delay the need for an actual new allocation box after some property change. This, however, means that e.g. the 'needs_allocation' flag never gets cleared, causing other parts of the code to get confused thinking it didn't pass through a layout step before paint.

Fix this by calling clutter_actor_set_allocation_internal() if there is still need for an allocation, after the allocation transition is created.