oplmusic: Handle key-on with volume 0 as key-off.

Some MIDI files, such as the music tracks in AV.wad, use a second
"key on" event with a volume of zero to mean "key off". Handle this
case correctly.
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.
'strife' is a valid game for chocolate-setup too!
Merge pull request #400 from rohitnirmal/setup-comment

'strife' is a valid game for chocolate-setup too!