Add a new pivot-point property to the ClutterAlignConstraint, similar to
the pivot point used by ClutterActor, defining the point in the
constraint actor around which the aligning is applied to the actor.
Just as the ClutterActor property, this property is defined using a
GraphenePoint.
By default this property remains set to (-1, -1) and the actor
will always be aligned inside the source actor, preserving the existing
behavior of ClutterAlignConstraint.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/737
