Accessibility: facilitate remapping of colours programatically and runtime-configurably using lua

[hiornso]

Plan: run remapping function once per colour/layer in composite maprender function, allowing the colour returned by the lua to be used by the bicubic function from a variable in C to make sure overhead is negligible.
Facilitated by a lua env that loads a script which provides a colour mapping function - can have some pre-installed (default, R-G colour-blindness) but also allow for custom ones to be written and loaded via CLI args.