shaders/colorspace: don't explode on luma > 1000
The current desaturation function extends linearly to infinity, resulting in very bad behavior for extreme HDR->SDR ratios (e.g. 49:1). Replace it by a logarithmic map to smooth this out more naturally at an asymptote.
This is more or less what the original code was intending, I think.
Fixes: 73785267