Add volumetric clouds3d renderer

Raymarches a horizontal cloud layer (28 steps, 4-octave 3D fBm).
Camera path is sinusoidal in altitude so the view dips into the
cloud deck at intervals for a 'fly through the edges' feel.
Density threshold from weather: sunny -> sparse, stormy -> overcast.
Renderer is selected via CONFIG.renderer ('clouds2d' or 'clouds3d').

background.html

@@ -474,12 +474,14 @@
     <script src="vendor/three.min.js"></script>
     <script src="vendor/vanta.clouds.min.js"></script>
     <script src="vendor/vanta.clouds2.min.js"></script>
+    <script src="vendor/clouds3d.js"></script>
     <script src="vendor/clouds2d.js"></script>
     <script>
 window.__clouds2dStarted = true;
 try {
 var noiseDataUri = 'data:image/png;base64,PLACEHOLDER';
-var useVanta = !(window.CONFIG && window.CONFIG.renderer === 'clouds2d');
+var renderer = (window.CONFIG && window.CONFIG.renderer) || 'vanta';
+var useVanta = (renderer !== 'clouds2d' && renderer !== 'clouds3d');
 var effect;
 if (useVanta && typeof VANTA !== 'undefined') {
     effect = (VANTA.CLOUDS2 || VANTA.CLOUDS)({
@@ -500,14 +502,16 @@ if (useVanta && typeof VANTA !== 'undefined') {
             texturePath: noiseDataUri
         }) || { setOptions: function() {} };
 } else {
+    // Pick the right renderer based on CONFIG.renderer
+    var _renderFn = (renderer === 'clouds3d' && window.clouds3dRender)
+        ? window.clouds3dRender
+        : window.clouds2dRender;
     effect = { setOptions: function() {} };
     function c2dRender() {
         var c2dCanvas = document.getElementById('vanta');
         if (!c2dCanvas) return;
-        c2dCanvas.width = window.innerWidth;
-        c2dCanvas.height = window.innerHeight;
-        if (typeof window.clouds2dRender === 'function') {
-            window.clouds2dRender(c2dCanvas);
+        if (typeof _renderFn === 'function') {
+            _renderFn(c2dCanvas);
         }
     }
     c2dRender();