st: Do not force allocation updates during shadow creation

If an actor is pending a relayout when get_allocation_box() is called,
the method forces an allocation update. In case of StWidget, this might
then result in a style update and a consecutive invalidation of the
shadow spec.

A helper method that invalidates one of its parameters as a side effect
(and by extension its return value as well) is most unexpected, so cur-
rently _st_create_shadow_pipeline_from_actor() poses an easy trap to
callers to run into.

Remove that trap by calling get_size()/get_position() instead, which
don't have the unintended side effect - it is still a good idea to fix
callers who were running into this to not waste resources on creating
shadows that are invalidated before the next paint, but throwing un-
defined behavior at them is harsh ...

https://bugzilla.gnome.org/show_bug.cgi?id=788908