So, this is a big mathdump with little in the way of history, but I
arrived at these algorithms after extensive testing, feedback and
research.

The basic approach is to construct a 3DLUT in IPTQc4 space. Inside the
3DLUT, we use a simplified, peak-dependent power-law relationship
between intensity and chromaticity in order to clip into the gamut hull,
with a brute force (bisection method) search.

Every attempt was made to provide approximately analogous equivalents to
the old PL_GAMUT_* modes, though the nature of this new infrastructure
means all results will be subtly different from before. In particular,
most of the colorimetric clipping modes (except "desaturate") will
prefer to slightly darken/brighten colors rather than desaturating them
towards white.

I could go on in more depth describing the new algorithms, but the code
should hopefully somewhat speak for itself.