[openal] B-Format support

[Chris Robinson]

As of just a little bit ago, OpenAL Soft supports B-Format buffer 
formats, with the AL_EXT_BFORMAT [1] and AL_EXT_MULAW_BFORMAT [2] 
extensions. These formats store a 2D (typical surround sound) or 3D 
sound field, and can be played back on about any conceivable speaker 
layout, like mono, stereo, 5.1, and more. This makes it a good format 
for better-than-stereo audio, like surround sound music or general game 
world ambiance. The formats also support rotation, so orientating the 
listener and/or source will rotate the sound (hopefully I've got the 
rotation matrix right..).

[1] https://icculus.org/alextreg/wiki/AL_EXT_BFORMAT
[2] https://icculus.org/alextreg/wiki/AL_EXT_MULAW_BFORMAT

If anyone feels like playing around with them, please do, and let me 
know if you encounter any issues. The one known issue is that this 
doesn't work very well with HRTF; such sources will currently just be 
rendered as plain stereo. I'm looking into ways to make it better, but 
using HRTF with Ambisonics seems to be a fairly complex issue.