shaders/colorspace: don't desaturate when raising brightness

Causes way more issues than it solves. A possible compromise could be to use the hull-shaped desaturation curve as well (but in the other direction), but looking at more and more SDR reference masters it seems like not having any desaturation at all is the way to go.

Fixes: https://github.com/mpv-player/mpv/issues/12707 Fixes: https://github.com/mpv-player/mpv/issues/15340