window-group: Decompose culling operations into two methods

This also fixes a bug in the translation of clip_region.

We will add an interface for this soon, so we can recursively cull like this...

https://bugzilla.gnome.org/show_bug.cgi?id=714706
window-actor: Move the operations we need to do when we cull out here

Soon, we'll move this into a generic MetaCullable interface, but for
now, just put hardcoded knowledge in MetaWindowGroup.

https://bugzilla.gnome.org/show_bug.cgi?id=714706
cullable: Turn cull_out / reset_culling into a separate interface

Instead of hardcoded knowledge of certain classes in MetaWindowGroup,
create a generic interface that all actors can implement to get parts of
their regions culled out during redraw, without needing any special
knowledge of how to handle a specific actor.

The names now are a bit suspect. MetaBackgroundGroup is a simple
MetaCullable that knows how to cull children, and MetaWindowGroup is the
"toplevel" cullable that computes the initial two regions. A future
cleanup here could be to merge MetaWindowGroup / MetaBackgroundGroup so
that we only have a generic MetaSimpleCullable, and move the "toplevel"
cullability to be a MetaCullableToplevel.

https://bugzilla.gnome.org/show_bug.cgi?id=714706