[BUG] Skincolor colormaps indexed using UINT8 instead of UINT16

When more than 256 skincolors are registered, slots beyond 255 are looping around to the start of the table of skincolors. This is verifiable by loading 86 or more custom skincolors at once, as combined with the 170 skincolors already present, this rolls past 255 and overflows to 0. Consequently, this allows players to use a copy of SKINCOLOR_NONE.

The maximum number of skincolors is defined as 1024 (due to NUMCOLORFREESLOTS). However, K_RainbowColormap and K_GenerateKartColormap (k_color.c) both only use UINT8. As it is only the color that is wrong, and not other properties, this is a potential cause.