Add a fast path for Ray Trace with OkLCh and linear RGB gamut mapping (#521)

We can't make global optimizations for every combination of perceptual
space and gamut when doing ray trace, but we can provide an optimized
path for the default (OkLCh perceptual space) and linear RGB spaces.
This is the path that CSS would take, and what we originally wrote the
approach for. While even faster approaches may end up making it into
CSS, Ray Trace is more flexible as a default here that handles gamut
mapping with various perceptual spaces, optimizing the default case
can improve general performance.

Resolves #520

Author: facelessuser