layout: Use a MetaBackgroundActor, not a custom ClutterX11TexturePixmap

While looking at how the plymouth implementation was built, I was so
short-sighted and focused on the string "_XROOTPMAP_ID" that I didn't
realize it was the name of the standard background on the root window.
Remove our own implementation, and switch to using a standard mutter
MetaBackgroundActor.

https://bugzilla.gnome.org/show_bug.cgi?id=682428