Reduces size used for png files from 19,034,446 bytes to 13,305,798 bytes in total. The smaller files also improve loading times as the zlib decoder has less bytes to schlep through.
The PNG decoder shipped with OpenRA only supports a bit depth of 8 for images with a palette, so must must ensure that other depths (1,2 or 4) are not used.
zopfilpng is used for compression with the following command line:
'zopflipng.exe --keepcolortype -y -m image.png image.png'
The keepcolortype flag ensures the bit depth is not changed.
* 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.
