Gtk3: wayland: Push NULL buffer when hiding a GdkWindow
This is the probably simplest fix for #4198, the "NULL buffer approach" from the issue. It re-applies the prior patch from Carlos.
Yes, I expect this will break xdg-shell v6 on mutter. I considered trying to restrict the change to xdgs stable only, but actually testing it would require checking out a circa 2017 version of mutter, so I did not.