atmosphere: declare uEarthRadius in sky shader

vendor/clouds3d.js

@@ -92,6 +92,7 @@ var FRAG_SRC = [
     'uniform float uAtmoDensity;',
     'uniform float uAtmoGrayTau;',
     'uniform float uAtmoGain;',
+    'uniform float uEarthRadius;',
     'uniform vec3 uAtmoBlueColor;',
     'uniform vec3 uAtmoGrayColor;',
     'uniform vec3 uSunColor;',     // direct beam after atmospheric extinction