clutter/actor: Use priv->parent instead of public API sometimes

The public API to get the parent actor, clutter_actor_get_parent() does a type check whether the actor is actually a ClutterActor. In case of _clutter_actor_apply_relative_transformation_matrix(), which is called recursively and very often during the paint process, this type check shows up with almost twice the amount of hits than the actual matrix multiplication.

So use the parent pointer directly in some code paths that are executed very often and avoid the expensive type checking there, we can do that since both places are not public API.