constraints: CSD windows need to have their titlebar kept onscreen too

GTK+ CSD windows are considered undecorated by the code, so we should
not force ourselves to only run on decorated windows.

https://bugzilla.gnome.org/show_bug.cgi?id=719772

src/core/constraints.c

@@ -1353,7 +1353,6 @@ constrain_titlebar_visible (MetaWindow         *window,
       window->type == META_WINDOW_DOCK    ||
       window->fullscreen                  ||
       !window->require_titlebar_visible   ||
-      !window->decorated                  ||
       unconstrained_user_action)
     return TRUE;