Landscape and Daytime gamesetup options for the Unknown, Danubius and Polar Sea, refs #4838.
Allow gamesetup options other than biome to determine the map peview image, refs #4962, rP21523.
Add previews for the 12 Unknown, 2 Danubius and 2 Polar Sea variations.
Concludes deduplication of the Unknown (rP12545), Unknown Land (rP12654) and Unknown Nomad (rP12747), refs #4317.
Differential Revision: https://code.wildfiregames.com/D2564