Allow driving the renderer from a web worker

The build generates a new output set called `render-webgl-worker`, meant
to be imported from a web worker. This exposes the `RenderWebGLRemote`
class to facilitate communication with the renderer using `postMessage`
and `onmessage`.
Only a few messages are implemented so far, but it's enough to run the
demo page.

webpack.config.js

@@ -4,7 +4,9 @@ var webpack = require('webpack');
 module.exports = {
     entry: {
         'render-webgl': './src/index.js',
-        'render-webgl.min': './src/index.js'
+        'render-webgl.min': './src/index.js',
+        'render-webgl-worker': './src/worker.js',
+        'render-webgl-worker.min': './src/worker.js'
     },
     devtool: 'source-map',
     output: {