win32: dnd should not be registerd for offscreen windows

https://bugzilla.gnome.org/show_bug.cgi?id=616544

gdk/win32/gdkdnd-win32.c

@@ -2330,6 +2330,9 @@ gdk_window_register_dnd (GdkWindow *window)
 
   g_return_if_fail (window != NULL);
 
+  if (gdk_window_get_window_type (window) == GDK_WINDOW_OFFSCREEN)
+    return;
+
   if (g_object_get_data (G_OBJECT (window), "gdk-dnd-registered") != NULL)
     return;
   else