Bug 1513 - Allow passing in ClutterPickMode to
clutter_stage_get_actor_at_pos()
At the moment, clutter_stage_get_actor_at_pos() uses CLUTTER_PICK_ALL
internally to find an actor. It would be useful to allow passing in
ClutterPickMode to clutter_stage_get_actor_at_pos(), so that the caller
can specify CLUTTER_PICK_REACTIVE as a criteria.