atmosphere: define EARTH_RADIUS_UNITS constant

vendor/clouds3d.js

@@ -2137,6 +2137,7 @@ var ATMO_MODEL_GRAY_TAU = (typeof ATMO_MODEL.grayTau === 'number')
     ? Math.max(0.1, Math.min(10.0, ATMO_MODEL.grayTau)) : 1.5;
 var ATMO_MODEL_GAIN = (typeof ATMO_MODEL.gain === 'number')
     ? Math.max(0.1, Math.min(5.0, ATMO_MODEL.gain)) : 2.0;
+var EARTH_RADIUS_UNITS = 6371.0 / KM_PER_TILE;
 var ATMO_MODEL_BLUE_COLOR = new Float32Array([0.22, 0.42, 0.95]);
 var ATMO_MODEL_GRAY_COLOR = new Float32Array([0.72, 0.76, 0.82]);
 // Integrated starlight + airglow. Physically this is about a millionth of