This is an indication value; the game tries to get as close as it
can, but due to the complex tropic rules, that is unlikely to be
exact.
In the end, it picks a height-level to base the desert/tropic
line on. This is strictly seen not needed, as we can convert any
tile to either. But it is the simplest way to get started with
this without redoing all related functions.
Setting the snow coverage (in % of the map) makes a lot more sense
to the human, while still allowing the niche player to set (by
finding the correct %) a snow line height they like. This makes for
easier defaults, as it decoupled terrain height from amount of snow.
Maps can never be 100% snow, as we do not have sprites for coastal
tiles.
Internally, this calculates the best snow line height to approach
this coverage as close as possible.
Without hills, not all industries can be generated, which means
that with a default configuration you get errors. This is far from
optimal, of course.
This now forces that there is at least some hills, even when you
are using very-flat. This is a stopgap solution, but a proper
solution requires a full rewrite of the terrain generator, which
is not a 2 minute (or even 2 week) job.
To make sure flat is still flat-ish, reduce the default
snow-line-height to 10, making it look a lot better on smaller maps.
This is a compromise between being able to have flat maps and
still having all industries on arctic.
