proxy encode: -c:v libx264 -c:a libopus -crf 28 -preset fast
encode: -c:v libx265 -c:a libopus -crf 18 -preset slow
remux: -c copy -map v:v? -map a:a? -map s:s?
remux video-only: -c copy -map v:v
remux audio-only: -c copy -map a:a