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
