Valleys mapgen code rewrite (#8101)

Shorter, simpler, clearer and more consistent with other mapgens,
while preserving functionality.
Base terrain shape is unchanged.
With the 'vary river depth' option disabled, river surface level
is unchanged.
Behaviour of the 4 heat/humidity/river depth options is very
slightly changed due to bugfixes and code cleanup (the mapgen is
'unstable').
Apply heat and humidity gradients above water_level instead of
above y = 0.