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]

[V3771n9]

If you want to use hardware mixing on Gus (max 1 mb), Interwave (max 16 mb) I could convert all sounds into instruments or patches using Awave (.pat), (.fff +.dat), or even for other cards (sf2, dls,etc). Let me know and I'll do it. As an option you could 1) use synth to play fx in hardware and digital out or CD tracks to play music; 2) or classic approach with software mix , or 3) hardware mixing like in vainilla doom till 1.22.
Note: When I used my GusPnP to live performance I notice that I must play notes that I am going to play (muted) to load patch caching before, so I do not have a delay while playing patches for the first time so I imagine I must be done something similar to load to memory the patches before playing a level.