Fix arctic biome with postrocessing and tweak for nubia and steppe
- Arctic biome was much too bright with postprocess (error in rP25757)
- Change sun angle in Nubia to make the grass less yellow at default camera angle
- Change sun angle in Steppe to make the game look less flat at default camera angle.
Patch by: marder
Differential Revision: https://code.wildfiregames.com/D4145