The empty OPL instrument check would check only the ADSR envelope fields for 0,
assuming this would mean the instrument would produce no sound. However, some
of these fields are flipped, i.e. max value would produce no sound. So this
shortcut was invalid. It is replaced by checking all fields for 0.
This adds a MIDI driver for AdLib / OPL2 and OPL3 with support for multiple
simultaneous sources of MIDI data. Additionally, it is easier to subclass the
driver and customize its behavior than the existing AdLib MIDI driver. The
standard behavior matches the Windows 95 SB16 driver.
