Rendering tests / add custom web worker loader

Also includes a change in the rollup-babel plugin to avoid
adding helpers as dependencies (which would give out errors).

rendering/webpack.config.js

@@ -22,5 +22,16 @@ module.exports = {
   context: __dirname,
   target: 'web',
   entry: entry,
-  devtool: 'source-map'
+  devtool: 'source-map',
+  module: {
+    rules: [{
+      test: /\.js$/,
+      use: {
+        loader: path.join(__dirname, '../examples/webpack/worker-loader.js')
+      },
+      include: [
+        path.join(__dirname, '../src/ol/worker')
+      ]
+    }]
+  }
 };

tasks/serialize-workers.js

@@ -20,6 +20,7 @@ async function build(input, {minify = true} = {}) {
     common(),
     resolve(),
     babel({
+      'externalHelpers': true,
       'presets': [
         [
           '@babel/preset-env',