avcodec: Add DFPWM reading support

This MR adds support for the DFPWM audio codec through WAV and raw DFPWM files. This codec was recently introduced to FFmpeg, but is currently not supported in VLC due to the codec ID being unknown. This MR adds a fourcc for DFPWM to the VLC ID list, and adds a conversion from the VLC fourcc to the libavcodec ID, allowing it to be handled automatically by FFmpeg.

More information on the DFPWM codec is available here: https://wiki.vexatos.com/dfpwm

This requires libavcodec 59.22 or later to function. The current version of FFmpeg's DFPWM codec has a bug in time reporting, which causes the file to end immediately in VLC; I've submitted a tiny patch to fix this, and I hope to have it merged within a few days.