This commit adds a feature to the multisource MIDI drivers which allows setting
a default value for most controllers. This default is then applied at the start
of every new track, which ensures controllers are in the correct state.
This can be used for games which use some controller, but do not reset it at
the start of each track, which can lead to incorrect playback.
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.
