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

src/shell-global.h

@@ -136,8 +136,6 @@ void     shell_global_reexec_self               (ShellGlobal  *global);
 
 const char *     shell_global_get_session_mode  (ShellGlobal  *global);
 
-ClutterActor * shell_global_create_xrootpmap_texture (ShellGlobal *global);
-
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */