Allow private class members to be mangled

Openlayers classes use an underscore suffix convention to mark class
members as private. These are undocumented implementation details and
presumably unsafe for code outside the class to rely on.

Allowing javascript minimisation to change these names gives 5% bundle
size reduction to the library.

config/webpack-config-legacy-build.js

@@ -1,4 +1,6 @@
 const path = require('path');
+const TerserPlugin = require('terser-webpack-plugin');
+
 module.exports = {
   entry: './build/index.js',
   devtool: 'source-map',
@@ -30,6 +32,16 @@ module.exports = {
       ol: path.resolve('./build/ol'),
     },
   },
+  optimization: {
+    minimizer: [
+      new TerserPlugin({
+        terserOptions: {
+          // Mangle private members convention with underscore suffix
+          mangle: {properties: {regex: /_$/}},
+        },
+      }),
+    ],
+  },
   output: {
     path: path.resolve('./build/legacy'),
     filename: 'ol.js',

examples/webpack/config.js

@@ -61,6 +61,10 @@ module.exports = {
         // Do not minify examples that inject code into workers
         exclude: [/(color-manipulation|region-growing|raster)\.js/],
         extractComments: false,
+        terserOptions: {
+          // Mangle private members convention with underscore suffix
+          mangle: {properties: {regex: /_$/}},
+        },
       }),
     ],
     runtimeChunk: {