Fix: Autoprefixer is now working (#16351)

The autoprefixer not working broke the phantomjs backend png rendering

Fixes #16345

.browserslistrc

+>1%,
+Chrome > 20
+last 4 versions,
+Firefox ESR

scripts/webpack/postcss.config.js

-module.exports = {
+module.exports = () => {
+  return {
     plugins: {
     plugins: {
-    'autoprefixer': {},
+      autoprefixer: {},
       'postcss-reporter': {},
       'postcss-reporter': {},
       'postcss-browser-reporter': {},
       'postcss-browser-reporter': {},
     }
     }
-}
+  };
+};

scripts/webpack/sass.rule.js

@@ -19,7 +19,7 @@ module.exports = function(options) {
         loader: 'postcss-loader',
         loader: 'postcss-loader',
         options: {
         options: {
           sourceMap: options.sourceMap,
           sourceMap: options.sourceMap,
-          config: { path: __dirname + '/postcss.config.js' },
+          config: { path: __dirname },
         },
         },
       },
       },
       {
       {