We should be passing around 24-bit samples (padded to 32-bits), rather than
16-bit. Places that need to change are:

1) MP3 decoder needs to shift right to 24 instead of 16.
2) FLAC decoder needs to shift left to 24.
3) PCM decoer needs to covert to 24.
4) Volume filter needs to switch to a 1.7 fixed format (from 1.15).
5) Visualizers need to shift right, but can remain essentially the same.
6) Audio transition code needs to change in the same way as the volume filter.