* Rename LeftColor and RightColor to MinColor and MaxColor
These are mapped from LowRadarColor and HighRadarColor in
the original inis, and appear to be used to set the bounding
values for selecting a random colour, NOT for left/right
pixels (which caused noticeably wrong banding).
* Adjust brightness based on terrain height.
MinHeightColorBrightness and MaxHeightColorBrightness
were chosen by trial/error to match the original
map preview rendering.
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