Add shell_util_get_transformed_allocation()

Add a function that gets the current allocation of an actor
transformed into stage coordinates. This avoids a misfeature of
clutter_actor_get_transformed_size() where when a size request is
queued (even if it won't eventually change the size), the returned
value is the transformed size request rather than the last allocation.

https://bugzilla.gnome.org/show_bug.cgi?id=645744