Screenshot HDR tone mapping

Whilst tone mapping never matches colours perfectly, and can be quite case-specific, I believe it's a significant upgrade over the normal washed-out colours you get when taking screenshots of HDR content with ffmpeg.

This can be achieved with the zscale filter in ffmpeg. Personally, I usually use the mobius algorithm, but there are a few to choose from, as well as an auto option provided by ffmpeg. Normally, I execute something like this:

ffmpeg -i "film.mkv" -vf "zscale=transfer=linear,tonemap=mobius,zscale=transfer=bt709,format=yuv420p" thumbnail.png

It could be implemented either as default (when HDR is detected) or as an option (e.g. --tonemap).