xwayland: Update screen pixmap for root window in xwl_window_set_pixmap

If the old window pixmap was the screen pixmap.

Fixes screen->GetScreenPixmap() returning a stale pointer to a destroyed
pixmap with rootful Xwayland. It would result in a crash after resizing
the Xwayland window, or at the latest when shutting down.

Fixes: 6779ec5bf6 ("xwayland: Use window pixmap as a window buffer")
Closes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1621

hw/xwayland/xwayland-window-buffers.c

@@ -281,6 +281,10 @@ xwl_window_set_pixmap(WindowPtr window, PixmapPtr pixmap)
 #endif
 
     TraverseTree(window, xwl_set_pixmap_visit_window, &visit);
+
+    if (window == screen->root &&
+        screen->GetScreenPixmap(screen) == visit.old)
+        screen->SetScreenPixmap(pixmap);
 }
 
 static PixmapPtr