As the RGB curves for many of the color rows are not linearly related,
my idea of scaling the brightest color in the row just didn't work.
Using a masked palette lookup works much better as it allows any curves.
Also, because the palette is uploaded as a grid and the coordinates are
calculated on the CPU, the system is extendable beyond 8-bit palettes.
This isn't quite complete as the top and bottom colors are still in
separate layers but their indices and masks can fit in just one, but
this requires reworking the texture setup (for another commit).
It turns out my approach to alias skin coloring just doesn't work for
the quake data due to the color curves not having a linear relationship,
especially the bottom colors.
