xaudio2: More than 256 audio sources

[Fabian Maurer]

Some applications run out of sources when using xaudio2 so I'm working on a 
patch that allows more than the 256 sources provided by OpenAL.

I've tried a few different methods, and the current should be the easiest and 
most compatible. Still, I'm not perfectly familiar with the implementation of 
xaudio2, and I ran into lots of hard to resolve issues, so I figured I'd ask 
for feedback before submitting the patch.
Any feedback or mistakes regarding my implementation?