Fix: Autoprefixer is now working (#16351)

The autoprefixer not working broke the phantomjs backend png rendering

Fixes #16345

(cherry picked from commit 2e59166daa)

scripts/webpack/postcss.config.js

@@ -1,7 +1,9 @@
-module.exports = {
-  plugins: {
-    'autoprefixer': {},
-    'postcss-reporter': {},
-    'postcss-browser-reporter': {},
-  }
-}
+module.exports = () => {
+  return {
+    plugins: {
+      autoprefixer: {},
+      'postcss-reporter': {},
+      'postcss-browser-reporter': {},
+    }
+  };
+};

scripts/webpack/sass.rule.js

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