While POSIX defines a single contiguous range of numbers that
determine a thread's priority. Win32 defines priority classes
and priority levels relative to these classes.
pthread maps those to -15 .. +15 with the top six ones
corresponding to REALTIME_PRIORITY_CLASS and max being
THREAD_PRIORITY_TIME_CRITICAL
Note that the PA backend can USE_MMCSS_THREAD_PRIORITIES
and PBD::MMCSS::set_thread_characteristics() directly for
the I/O threads.
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