When in a partial stage paint, we can combine that with the visibility
information in MetaWindowGroup to further eliminate unneeded drawing.
Since there is no current Clutter API to access the current clip,
drop to using GL directly.
https://bugzilla.gnome.org/show_bug.cgi?id=634779
Move all objects and functions namespaced with Mutter into the Meta namespace
to get a single consistent namespace. Changes that aren't simply changing mutter
to meta:
MutterWindow => MetaWindowActor
mutter_get_windows => meta_get_window_actors
mutter_plugin_get_windows => meta_plugin_get_window_actors
https://bugzilla.gnome.org/show_bug.cgi?id=628520