Add code to track and draw the root window background. The advantage of doing
it here as compared to in a plugin is that we can use the visiblity smarts
of MetaWindowGroup to optimize out drawing the background when obscured.
If handling other than tracking the _XROOTPMAP_ID property is desired in the
future, more functionality like setting the background from a file or doing
cross-fades can be added.
The new background actor is exposed to plugins via meta_plugin_get_background_actor()
similar to other exposed actors to allow cloning the background for use in
other displays. The actual class is not installed for public consumption at
the moment since it has no useful methods.
https://bugzilla.gnome.org/show_bug.cgi?id=634833
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
