* Move function to compute visible screen layers to BKE.
* Use this now in the depsgraph, was still using this all layers
to flush. Still missing a way to get the current scene in
background mode..
* Also two more function to not require a scene pointer anymore:
* DAG_object_update_flags is now DAG_id_update_flags.
* DAG_ids_flush_update is now available next to DAG_scene_flush_update.