Re-enable hardware mixing on Gravis Ultrasound cards

[viti95]

The Apogee Sound System uses software mixing instead of hardware mixing, which is slower but provides more functionality. Add the option to use hardware mixing.

7/24/95

• Changed GUS code to use software mixing instead of hardware mixing.
  GUS purists will hate me (if they found out), but it makes a lot more
  sense this way since I only have to maintain one playback method, plus
  it allows GUS owners to hear reverb and gives better control over panning.
  There may be other benefits, but I can't think of them right now.

[viti95]