Two sanity checks to make sure the debug env vars still work are added too. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2456>
This allows different types of backends to coexist, would e.g. one device support atomic but another not. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1488>