atmosphere: keep Rayleigh sky behind terrain and make haze neutral

vendor/clouds3d.js

@@ -368,8 +368,7 @@ var FRAG_SRC = [
                 'float dens = exp(-alt / Hs) / Hs;',
                 'float dtau = uAtmoDensity * dens * step;',
                 'float transBefore = exp(-tau);',
-                'float grayMix = clamp(tau / max(0.01, uAtmoGrayTau), 0.0, 1.0);',
-                'vec3 atmoCol = mix(uAtmoBlueColor, uAtmoGrayColor, grayMix);',
+                'vec3 atmoCol = uAtmoGrayColor;',
                 'vec3 light = uSunColor * uSunInt * sunPhase + uMoonColor * uMoonInt * moonPhase * 2.0 + max(vec3(uSkyAmb), uStarAmb * vec3(0.7, 0.85, 1.0)) * 0.65;',
                 'scatter += atmoCol * light * dtau * transBefore;',
                 'tau += dtau;',