[Openal] ADPCM support

[Garin HIEBERT]

There is an AL_EXT_IMA4 extension for handling IMA4 ADPCM, but support is
far from universal, so you can't really count on it being there.  As you
said, there was certainly talk of doing on-the-fly decompression in the
past, and some experiments with doing so (I did one of them with Ogg
Vorbis).  It seems to have worked out since then that people have been
doing their own thing, independent of OpenAL -- either decomressing the
whole file and dumping it into a single buffer, or decompressing in smaller
chunks into queued buffers.

Garin