Since for mpv CLI, the player state is a singleton, full prefetching is
a bit tricky. We do it only on the demuxer layer.
The implementation reuses the old "open thread". This means there is
significant potential for regressions even if the new option is not
used. This is made worse by the fact that I barely tested this code.
The generic mpctx_run_reentrant() wrapper is also removed - this was its
only user, and its remains become part of the new implementation.
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