SystemMenu: wait for a completed paint before switching VT

Activating the GDM login screen switches VT and causes X to freeze
event processing (because it lost the drm master), so must make
sure to have painted the lock screen at least once before proceeding,
or the user can go back and see the unlocked desktop.

https://bugzilla.gnome.org/show_bug.cgi?id=708051

js/ui/status/system.js

@@ -372,7 +372,11 @@ const Indicator = new Lang.Class({
         Main.overview.hide();
         if (Main.screenShield)
             Main.screenShield.lock(false);
-        Gdm.goto_login_session_sync(null);
+
+        Clutter.threads_add_repaint_func_full(Clutter.RepaintFlags.POST_PAINT, function() {
+            Gdm.goto_login_session_sync(null);
+            return false;
+        });
     },
 
     _onQuitSessionActivate: function() {