This patch further changes saturation filter after D14808 by using HSL instead of HSV and using lightness as a mask instead of existing saturation.
This results in saving highlighted areas of the image, instead of losing detail like before.

Things to note:

1. This is an experiment and we shouldn't merge it as is
2. It will be best to add this as an option for the filter, I'll update the patch to do that later
3. Old variable name for color is hsv_color, which doesn't make sense if saturation uses hsl. Will probably need a new one, maybe temp_color?

Video

Testfile