Fix arctic biome with postrocessing and tweak for nubia and steppe

- Arctic biome was much too bright with postprocess (error in
717a9d0252)
- 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
This was SVN commit r25764.

binaries/data/mods/public/maps/random/rmbiome/generic/arctic.json

@@ -24,7 +24,7 @@
 		},
 		"Postproc": {
 			"PostprocEffect": "hdr",
-			"Brightness": 1,
+			"Brightness": 0,
 			"Saturation": 1,
 			"Contrast": 1,
 			"Bloom": 0.16

binaries/data/mods/public/maps/random/rmbiome/generic/nubia.json

@@ -6,7 +6,7 @@
 	"Environment": {
 		"SunColor": { "r": 1.10039, "g": 1.05979, "b": 0.99212 },
 		"SunElevation": 0.7,
-		"SunRotation": -0.3,
+		"SunRotation": -1.7,
 		"AmbientColor": { "r": 0.354118, "g": 0.361961, "b": 0.389412 },
 		"Water": {
 			"WaterBody": {

binaries/data/mods/public/maps/random/rmbiome/generic/steppe.json

@@ -7,7 +7,7 @@
 		"SkySet": "sunrise",
 		"SunColor": { "r": 0.85866, "g": 0.85095, "b": 0.73284 },
 		"SunElevation": 0.421456,
-		"SunRotation": -0.226047,
+		"SunRotation": -1.526047,
 		"AmbientColor": { "r": 0.504118, "g": 0.462157, "b": 0.604118 },
 		"Water": {
 			"WaterBody": {