win32: Avoid type warning for offscreen windows in xp theme

See bug #598299

gdk/win32/gdkwin32.h

@@ -73,6 +73,9 @@ G_BEGIN_DECLS
 #endif
 
 
+/* Return true if the GdkWindow is a win32 implemented window */
+gboolean      gdk_win32_window_is_win32 (GdkWindow *window);
+
 /* Return the Gdk* for a particular HANDLE */
 gpointer      gdk_win32_handle_table_lookup (GdkNativeWindow handle);
 

gdk/win32/gdkwindow-win32.c

@@ -3378,6 +3378,12 @@ gdk_window_impl_iface_init (GdkWindowImplIface *iface)
   iface->destroy = _gdk_win32_window_destroy;
 }
 
+gboolean
+gdk_win32_window_is_win32 (GdkWindow *window)
+{
+  return GDK_WINDOW_IS_WIN32 (window);
+}
+
 GdkDrawable *
 gdk_win32_begin_direct_draw_libgtk_only (GdkDrawable *drawable,
 					 GdkGC *gc,