[openal] [WIP] AL_EXT_MSADPCM in OpenAL Soft

[Chris Robinson]

I've added two extensions to OpenAL Soft in git: AL_SOFT_block_alignment 
and AL_SOFT_MSADPCM.

AL_SOFT_block_alignment adds block alignment properties for unpacking 
(reading from user) and packing (writing to user) buffer data. The 
alignments are specified in sample frames. Samples can only be read or 
written in complete blocks. It also, obviously, affects how ADPCM is 
decoded and encoded. An alignment of 0 means a default is used... 
currently this is 65 for IMA4 (to maintain compatibility with existing 
code), 64 for MSADPCM, and 1 for other PCM formats.

AL_SOFT_MSADPCM adds AL_FORMAT_MONO_MSADPCM_SOFT and 
AL_FORMAT_STEREO_MSADPCM_SOFT buffer formats, as described here:
http://wiki.multimedia.cx/?title=Microsoft_ADPCM
Pretty self explanatory. It normally uses a block alignment of 64 sample 
frames, but this can obviously be changed with the 
AL_SOFT_block_alignment extension.

The exact behavior of the extensions isn't yet set in stone, and can be 
modified based on any feedback (particularly the default MSADPCM block 
alignment, if it really makes any difference). I can also change the 
"SOFT" to "EXT" if anyone thinks it would be a good idea to, I just 
never feel right about using EXT without some general consensus from 
implementors about an extension.

I haven't yet put in any optimizations, such as avoiding the temp buffer 
when dealing with signed 16-bit buffer data. But please test it out, let 
me know how it works, and feel free to provide feedback. :)