opl: Process MIDI 'set tempo' meta events.

The MIDI format includes a special meta event to set the tempo of
playback, and some WADs depend on this - notably the music in Alien
Vendetta. Move the variables controlling tempo to the global scope
(they are not per-track as I previously thought) and set when the
tempo events are encountered.

This is some progress towards resolving #334, but that bug is not
yet completely fixed, because the tempo change does not
retroactively apply to OPL timers that have already been set.