[openal] [WIP] AL_EXT_MSADPCM in OpenAL Soft

[Ethan Lee]

Hello!

Over the last few months I've been working on support for MSADPCM 
wavedata in OpenAL Soft. The primary motivation for this work was 
improving support for XACT WaveBank files in MonoGame-SDL2, an open 
source XNA4 reimplementation:

https://github.com/flibitijibibo/MonoGame

Before pursuing this work further, I wanted to ask the mailing list for 
feedback regarding what we have now. At the moment we have support for 
decoding MSADPCM wavedata, but no support for encoding (those portions 
are currently stubbed, but if this is simply not needed then I wouldn't 
object to removing it). Additionally, we only support 64-/128-/256-byte 
blocks, and each have their own code paths. I wasn't sure if this was 
the best path, or if there should be a new BufferData function that 
accepts a `blockAlign` parameter to handle a wider range of block sizes 
in addition to possibly reducing the amount of code.

The current patch is here:

http://flibitijibibo.com/al_ext_msadpcm.txt

And the Git repository is here:

https://github.com/flibitijibibo/openal-soft-msadpcm/


Any feedback is appreciated!

-Ethan