Implement some basic audio mixing
Implement auto output resampling (if requested)

If there are for example more than one audio outputs and they have
different sample rates or channels and such, this will allow automatic
conversion of that audio to the request formats/channels/rates (but only
if requested).
Add Libavutil dependency for obs-ffmpeg plugin

av_frame_alloc, av_frame_free, among others, live in libavutil