- Automatically write a mapcolors.txt file for use with
  minetestmapper if infodump features enabled.
- Define colors for terrain mod and "below" lexically,
  and for anything that inherits from it.

This will allow mods to register colors to use on the
mapper automatically, and benefit from inheritance
where material inheritance is in use, rather than
having to have each mod distribute its own text
file and mapper users having to collect them all
together manually.