mutter needs GDK to use the x11 backend. It already calls
gdk_set_allowed_backends ("x11") for this purpose; however, if
GDK_BACKEND=wayland (or any other non-x11 backend possibly) happened to
be in the environment, GDK would fail to initialize at all. This would
result in mutter not registering as X11 window manager, and all X11
clients hanging.
Big thanks to Olivier Fourdan for figuring this out!
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=2022283 Bug: https://bugs.debian.org/1008992