pt3_lib, but playing 6 channels (on both AY-3-8910) of the Mockingboard

Vortex Tracker supports this, essentially just playing two pt3 files at once

this is a hack, just duplicating all the player code.

It would probably be possible to share more common code.

Note: it assumes the songs you are playing are similar.  While it would
	be possible to set it up so they could use separate note tables,
	it seems odd and we currently don't support it.

