I'm not sure if this option affects anything or if it's a placebo,
especially since the VO thread is now registered with MMCSS. Still, I
think --priority=high may have helped back when I used mplayer2 on a
netbook. It's also possible that encoding-mode users would want to set
--priority=idle.
Anyway, it was one of the last M_OPT_FIXED options, so fix that.