shaders/icc: lower lcms2 version dependency

This is a quick hack. In theory, we should replicate the logic from the function. Actually, in looking up its implementation, I realized that calling cmsDetectRGBProfileGamma multiple times is fairly wasteful, and we should re-use the generate tabulated tone curves instead.

Indeed, we should even re-use the cmsEstimateGamma result if possible, somehow.