perf: reduce cloud shadow samples

vendor/clouds3d.js

@@ -1053,10 +1053,11 @@ var TERRAIN_FRAG_SRC = [
                 'if (tBot > 1e8) { tBot = 0.0; tTop = 14.0; }',
                 'if (tTop > 0.0 && tTop > tBot && tTop < 90.0) {',
                     'float dens = 0.0;',
-                    'for (int ci = 0; ci < 5; ci++) {',
-                        'float tt = mix(tBot, tTop, (float(ci) + 0.5) / 5.0);',
+                    'for (int ci = 0; ci < 3; ci++) {',
+                        'float tt = mix(tBot, tTop, (float(ci) + 0.5) / 3.0);',
                         'dens += cloudDensity(vWorld + uSunDir * tt, baseTh, smoothstep(4.0, 18.0, tt));',
                     '}',
+                    'dens *= 5.0 / 3.0;',
                     'cShadow = mix(uCloudShadowFloor, 1.0,',
                         '    exp(-dens * uCloudShadowDensity));',
                 '}',