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.c

@@ -1693,43 +1693,3 @@ shell_global_get_session_mode (ShellGlobal *global)
 
   return global->session_mode;
 }
-
-/**
- * shell_global_create_xrootpmap_texture:
- * @global: The #ShellGlobal
- *
- * This returns the _XROOTPMAP_ID pixmap that gdm should have stuffed
- * in the root window. The goal here is to allow a smooth fade between
- * plymouth and the shell greeter. This is also a workaround for gjs not
- * supporting raw xlib types.
- *
- * Returns: (transfer floating): A #ClutterActor that represents the
- * _XROOTPMAP_ID pixmap property from the root window.
- */
-ClutterActor *
-shell_global_create_xrootpmap_texture (ShellGlobal *global)
-{
-  Atom res_type;
-  int res_format;
-  unsigned long res_nitems, bytesafter;
-  unsigned char *data;
-  Pixmap root_pixmap_id = None;
-
-  g_return_val_if_fail (SHELL_IS_GLOBAL (global), NULL);
-
-  if (XGetWindowProperty (global->xdisplay,
-                          DefaultRootWindow (global->xdisplay),
-                          XInternAtom (global->xdisplay, "_XROOTPMAP_ID", False),
-                          0, G_MAXLONG, False, XA_PIXMAP,
-                          &res_type, &res_format, &res_nitems, &bytesafter, &data) == Success)
-    {
-      if (res_type == XA_PIXMAP && res_format == 32 && res_nitems == 1)
-        root_pixmap_id = * (Pixmap *) data;
-      XFree (data);
-    }
-
-  if (root_pixmap_id != None)
-    return clutter_x11_texture_pixmap_new_with_pixmap (root_pixmap_id);
-  else
-    return NULL;
-}

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__ */