This is an awful corner-case party, because we've started to allow the
user to send track-switching related commands before stream selection is
done in the loading stage. If mp_switch_track() is called before this
stream selection, it can behave inconsistently. So if we're in the
pre-loading phase, we must set the stream selection options to get
streams selected later, instead of calling this function. There seem to
be some annoying circumstances that exclude mp_switch_track() from
handling this logic too, so do it at all call-sites.
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