Surround Sound

[SirNate0]

Presently we only support mono and stereo audio. SDL seems to support up to 7.1 surround sound, and I don't think it would be that much work to add support for it.

• Opening the sound device would become a little more complicated, as we presently open stereo and fall back to mono if that fails. I think we would just replace this with decreasing the number of channels by 1 until we get to mono or succeed in opening the device with whatever the user requested.
• Mixing would need to be rewritten. Presently we have separate functions for each combination of (from mono|stereo)(to mono|stereo)(interpolated or not). I think we could probably rewrite these into only a couple functions with template parameters specifying these (from channels, to channels, interpolated).
• Panning would need an additional dimension for Front/Back. And while we are at it, we might want to just make it a Vector3 to support full spatial audio in the future. Presently it is only possible to pan a mono source sound, and I think it should remain this way, and stereo/surround sound audio should be handled as is (the user can always separate multi channel audio into mono tracks if they really need the per-channel panning control).