More cloud structure: 4 fBm layers, 4 octaves each

Density reverted to original 0.20/0.70/1.00/0.45 but structure
denser than ever:
- fbm3: 3 -> 4 octaves (more detail at fine scales)
- cloudDensity: 4 layers (base + mid 35% + fine1 18% + fine2 10%)
- smoothstep 0.24 -> 0.20 for tighter cloud edges
Cost: 4 fbm * 4 octaves = 16 noise3 samples per step * 18 steps
= 288 samples per pixel. Watch FPS on low-end GPUs.

vendor/clouds2d.js

@@ -122,10 +122,10 @@ function hex(rgb) {
 }
 
 var WEATHER_DENSITY = {
-    sunny: 0.65,
-    cloudy: 0.90,
+    sunny: 0.20,
+    cloudy: 0.70,
     stormy: 1.00,
-    night: 0.75
+    night: 0.45
 };
 
 // Load the noise image and upload it as a REPEAT-wrapped texture.