It's the impl side that wants to add impl side idle sources, or fd
sources, etc, so make it part of MetaThreadImpl.
This changes things to be GAsyncQueue based. While things are still
technically single threaded, the GAsyncQueue type is used as later we'll
introduce queuing tasks asynchronously, then eventually queuing across
thread barriers.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>
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>
