[mutter] stage/x11: Get root window from the X11 backend

[Marge Bot <marge-bot src gnome org>]

commit cf00ee9724319dcd45d1fdb64cf8398d7ca99631
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Sep 17 21:37:08 2021 +0200

    stage/x11: Get root window from the X11 backend
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2014>

 src/backends/x11/meta-stage-x11.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/src/backends/x11/meta-stage-x11.c b/src/backends/x11/meta-stage-x11.c
index 388349ca1e..8caddeb547 100644
--- a/src/backends/x11/meta-stage-x11.c
+++ b/src/backends/x11/meta-stage-x11.c
@@ -601,10 +601,14 @@ handle_wm_protocols_event (MetaStageX11 *stage_x11,
   else if (atom == clutter_backend_x11->atom_NET_WM_PING &&
            xevent->xany.window == stage_x11->xwin)
     {
+      MetaBackend *backend =
+        meta_stage_impl_get_backend (META_STAGE_IMPL (stage_x11));
+      MetaBackendX11 *backend_x11 = META_BACKEND_X11 (backend);
+      Display *xdisplay = meta_backend_x11_get_xdisplay (backend_x11);
+      Window root_xwindow = meta_backend_x11_get_root_xwindow (backend_x11);
       XClientMessageEvent xclient = xevent->xclient;
-      Display *xdisplay = xdisplay_from_stage (stage_x11);
 
-      xclient.window = clutter_backend_x11->xwin_root;
+      xclient.window = root_xwindow;
       XSendEvent (xdisplay, xclient.window,
                   False,
                   SubstructureRedirectMask | SubstructureNotifyMask,