At the moment if a thread is made real-time there's no going back,
it stays real-time for the duration of its life.
That's suboptimal because real-time threads are expected by RTKit to
have an rlimit on their CPU time and certain GPU drivers in the kernel
can exceed that CPU time during certain operations like DPMS off.
This commit adds two new ref counted functions:
meta_thread_{un,}inhibit_realtime_in_impl
that allow turning a thread real-time or normally scheduled. At the same
time, this commit stores the RTKit proxy as private data on the thread
so that it can be reused by the above apis.
A subsequent commit will use the new APIs.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3324>
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