GtkWindow: Manually set CSD should enable shadow width calculation

If CSD is enabled with shadow even though it "shouldn't"*, the width
should still be calculated correctly. This fixes a regression caused by
b1e5ad469c.

* gtk_window_should_use_csd () returns false

https://bugzilla.gnome.org/show_bug.cgi?id=748615

gtk/gtkwindow.c

@@ -6542,7 +6542,7 @@ get_shadow_width (GtkWidget *widget,
   *shadow_width = border;
 
   if (!priv->decorated ||
-      !gtk_window_should_use_csd (window) ||
+      (!gtk_window_should_use_csd (window) && !priv->client_decorated) ||
       !gtk_window_supports_client_shadow (window))
     return;