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