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.
At least it seems to me that the way e.g. Lure of the Temptress calls
isReady() to see if the driver has finished processing all the custom
sounds, _sysExQueue can be accessed by two threads simultaneously.
Which seems like a bad thing to me!
