clutter/actor: Use priv->allocation instead of get_allocation_box()

The comment in _clutter_actor_get_allocation_clip() explicitely notices that it doesn't need the behavior of doing an immediate relayout as clutter_actor_get_allocation_box() does. The comment is also still valid since the code calling _clutter_actor_get_allocation_clip() checks for priv->needs_allocation just before.

So let's just use the allocation directly here instead of going through that function.