Fix undecorated windows

Setting windows undecorated was broken by some of the recent
shadow width changes. We need to ensure that shadow width is
zero for undecorated windows, then things work again.

gtk/gtkwindow.c

@@ -6562,7 +6562,8 @@ get_shadow_width (GtkWidget *widget,
 
   *shadow_width = border;
 
-  if (!priv->client_decorated)
+  if (!priv->decorated ||
+      !priv->client_decorated)
     return;
 
   if (priv->maximized ||