[openal] [WIP] AL_EXT_MSADPCM in OpenAL Soft

[Ethan Lee]

To be honest I don't have my heart set _too_ much on the performance 
improvements, though some of them really are amazingly huge 
improvements. A lot of why that's there is because of how XNA loads 
WaveBank data; non-streaming WaveBanks just shove all the data into RAM 
at once, so when we're doing this in MG-SDL2 we're just converting _all_ 
of this stuff at once... you'll notice in a few of my ports how long it 
takes just to get to the splash screen. >_<

I'm also totally down to get rid of all the blocksize enums; it's both 
restrictive and is even a source of boogs when copypasting code to add a 
new block size. Plus, so many wasted #define values...

It'd probably be a pain to work that into the library without stomping 
on the other SOFT extensions, but at least it'd be to the benefit of 
multiple formats.

-Ethan