opl additive voice volume calculation fix

opl additive voice volume calculation fix
opl additive voice volume calculation fix

opl additive voice volume calculation fix
Merge pull request #465 from khokh2001/opl-fix

opl additive voice volume calculation fix

Adjust how the OPL volume register values are calculated based on the
channel, note and global MIDI volume, to better match how the Doom
DMX library performs these calculations (thanks Alexey Khokholov /
khokh2001).