clouds: raise cloud top to 20 km and disable terrain occlusion for test

vendor/clouds3d.js

@@ -6855,7 +6855,7 @@ window.clouds3dRender = function(target) {
         gl.activeTexture(gl.TEXTURE7);
         gl.bindTexture(gl.TEXTURE_2D, terrainDistTex || null);
         gl.uniform1i(s.uTerrainDistTex, 7);
-        gl.uniform1f(s.uTerrainDistOk, (terrainDistOk && terrainDistTex) ? 1.0 : 0.0);
+        gl.uniform1f(s.uTerrainDistOk, 0.0); // test: clouds ignore terrain occlusion
         gl.disable(gl.DEPTH_TEST);
         gl.depthMask(false);
         gl.enable(gl.BLEND);