This is a central lock (that is to stay and has no reason to go away),
and it was simply made global. This reduces complexity when the original
MPlayer code was changed from single thread + global state to a context
handle.
Having the global lock was still a bit silly if there were multiple mpv
instances in the process, because it would make the instances wait for
each other for no reason. So move it to the per-instance context, which
is trivial enough.
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