This uses the queue that was introduced when migrating impl task
management from MetaThread to MetaThreadImpl, with the exception that
it's now fully used as an actual queue. It now has a GSource that sits
on the right GMainContext that is dispatched whenever there are tasks to
execute.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>
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>
