[Openal] Lots of sources

[Garin Hiebert]

With the software-mixing paths (DS and MMSYSTEM), the limit should be 32
voices.  That was arbitrarily picked to keep CPU usage under control. 
With the DS3D path, all the card's hardware voices should be supported
without an upper limit (if the card doesn't have at least 16 real
hardware voices, then the implementation switches to a DirectSound backend).

With newer soundcards like the Sound Blaster Audigy 2, you should end up
with 62 voices whether using the native OpenAL library or the DS3D
wrapper code.

Garin