Sample-exact models: InstrumentTrack vol/pan knobs s.ex. support

This was a bit tricky because instrumentTracks are inherently such that they may contain an offset, which would mean that the models wouldn't match up with the instruments.
I solved this by passing the ValueBuffers all the way from InstrumentTrack, via Mixer, to AudioPort, where the offset is compensated for, and doing the actual vol/pan modulation there. I think it's a pretty neat solution and seems to work fine, but this should be reviewed just to be safe, just to see that I'm not doing anything really stupid here...