[Cin] ac4 and ffmpeg

Probably not very relevant right now but apparently Dolby pushed their new ac4 codec as part of new digital TV standart.. so.. it might be encountered in new broadcasts. ffmpeg decoder was already written, but not in any release or even main tree see https://trac.ffmpeg.org/ticket/8349 and last comment pointing at recent-ish rebase https://github.com/makrsmark/FFmpeg/commits/ac4 if I understand correctly only mpeg transport stream currently supported as container, and not, say, mp4.