environment: Disallow all GDK backends
There is only a very limited subset of GTK that is usable from within the compositor, and by now we should have replacements for all of them in place.
On the other hands there is plenty that can go catastrophically wrong in the rest. In particular on wayland, GTK must never open a wayland display connection.
Make sure that extensions don't do anything silly, by disallowing all GDK backends.