cullable: Reset the culling state instead of skipping the traversal

When we traversed down to reset the culling state, previously we
would just skip any actors that wanted culling. In order to properly
reset the unobscured_region before painting, we need to traverse down
to these places as well. Do this by calling cull_out with NULL regions
for both arguments, and adapt existing cull_out implementations to
match.

https://bugzilla.gnome.org/show_bug.cgi?id=720631
shaped-texture: Move unobscured_region processing here

We want to remove a bunch of auxilliary duties from the MetaWindowActor
and MetaSurfaceActor, including some details of how culling is done.
Move the unobscured region culling code to the MetaShapedTexture, which
helps the actor become "more independent".

https://bugzilla.gnome.org/show_bug.cgi?id=720631