Use the new "background actor" functionality in Mutter

The code to draw the root background has now been moved into Mutter,
with added smarts to not draw obscured portions. Remove the old
version of the code and clone the Mutter background actor to draw
the background in the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=634836

js/ui/workspace.js

@@ -301,7 +301,7 @@ DesktopClone.prototype = {
     _init : function(window) {
         this.actor = new Clutter.Group({ reactive: true });
 
-        let background = new Clutter.Clone({ source: Main.background.source });
+        let background = new Clutter.Clone({ source: global.background_actor });
         this.actor.add_actor(background);
 
         if (window) {