window: Don't move the desktop window after monitor hotplug

If someone plugs in a new monitor, while all their regular windows
should move in absolute X coordinates to ensure they stay on the
same monitor, the desktop window should stay put.

https://bugzilla.gnome.org/show_bug.cgi?id=681159

src/core/window.c

@@ -4454,6 +4454,9 @@ meta_window_update_for_monitors_changed (MetaWindow *window)
   const MetaMonitorInfo *old, *new;
   int i;
 
+  if (window->type == META_WINDOW_DESKTOP)
+    return;
+
   old = window->monitor;
 
   /* Start on primary */