Make minimap height levels color modifiable by paksets

[Nazalassa]

Currently, the height levels of the minimap are colored with shades of green (or shades of blue for water areas). It could be nice if paksets were able to change these colors (with e.g a specific entry in simuconf.tab) as some paksets feature non-green ground, and/or non-blue water. For example, the pak48.bitlit could use shades of grey for land, as it would fit better with its grey ground; and the pak48.Excentrique could use rainbow colors or something more excentrique.

[Yona-TYT]

I think this is a very interesting idea, but there are many colors in Simutrans that may be customizable in the future, I think the list of parameters will be very long, or not?.