shaders/colorspace: improve PL_GAMUT_WARN algorithm

Instead of highlighting after 0.5% of peak delta, highlight at a very generous threshold (1e-6). To counteract the negative effects of transforming a bunch of slightly-out-of-gamut dark pixels into bright pink, change the highlighting algorithm to simply rotate the hue vector by 180°

This should be strictly more noticeable than the previous while also allowing you to still see what's being highlighted, as an added benefit.

Fixes: #239 (closed)