- Split The Last Express' ADPCM to the engine. Using the MS IMA routine was really a hack.
- Fixed stereo MS IMA ADPCM, the old routine was completely wrong.
There are tons of ADPCM variants out there, and it is impractical to
stuff them all into a single adpcm.cpp file. By exposing the internals,
engines can implement their ADPCM decoder variants more easily.