The COM-style interface to a MIDI driver.
This is the function table interface to a MIDI driver. Both version 1 and 2 drivers use
this same table of function pointers (except as noted).
Drivers which support both the version 1 and version 2 interfaces can tell which version
of the server is running by checking to see whether kMIDIDriverInterface2ID or
kMIDIDriverInterfaceID is passed to the factory function. If the version 1 interface is
requested, the driver should behave as if it is a version 1 driver.