[Cin] HDR -> sdr tonemap on cpu only

[Andrea paz]

As far as I know, it is not advisable to use LUTs for color space
conversion. They are good for the “color look.” Better to do it by
hand. To get an idea of the steps to be taken (even if they cover the
opposite case, i.e. SDR --> HDR, they are still rich in information):

https://sourceforge.net/projects/qtpfsgui/
https://codecalamity.com/encoding-uhd-4k-hdr10-videos-with-ffmpeg/
Also:
https://www.youtube.com/watch?v=7nI6VPTfvok

In CinGG you could use “3 Color Way,” but it has the limitation of the
“Value” slider.


PS: I had recently made additions to the HDR manual; the last 2
commits. Could you please check and correct/delete the wrong parts? I
am delving into the subject, but still confused.