properly compute colors for palette indexes above 15 in curses builds

[db48x]

Summary

Bugfixes "avoid UB by properly computing colors for palette indexes above 15 in curses builds"

Purpose of change

Fixes #72973, #76092

Describe the solution

ANSI defines the default RGB values for color indexes above 15 using a simple formula, so just do that. While I was there, I went ahead and cached all the values avoid having to do division every time a color is needed from the palette. Probably not a huge cost savings overall, but it feels better.

Testing

Display any uilist with a disabled item in a curses build. Easiest way to do this is use “Play Now!” to start a new game, then walk into the same tile as the NPC who is with you in the evac shelter.

[l29ah]

Built it, it works well, thank you.
By the way, is there a way to set the menu backgrounds back to black and the text back to white? Gray on gray isn't very readable.

[GuardianDll]

Restarting borked tests