It currently does exactly what MetaKms and MetaKmsImpl did regarding the
context separation, which is to isolate what may eventually run on a KMS
thread into a separate unit. It works somewhat like a "user thread",
i.e. not a real thread, but will eventually learn how to spawn a
"kernel thread", but provide the same API from the outside.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>
