ADPCM playback was broken by #991 when an older FFmpeg version (< 3.1)
was used. The newer version has a better API for handling decoder that
return multiple frames of data per packet which I already used. However,
I didn't bother implementing that for the older API since I haven't seen
a decoder that actually returned multiple frames per packet but ADPCM is
such a decoder. This fixes that by implementing the correct behavior
with the old API.
This is the first step towards moving all audio decoding to FFmpeg which
will reduce the amount of external code we will need to include. This
only replaces the AudioStream code with an FFmpeg based solution which
should make the code easier to review.
asarium