Let Pico::getTwigVariables() return the "real" array

Currently Pico::getTwigVariables() always returns the default twig variables and ignores all additions/changes made through the onPageRendering event. The method now returns the "real" variables array used by Twig.

lib/Pico.php

@@ -1474,6 +1474,10 @@ class Pico
      */
     protected function getTwigVariables()
     {
+        if ($this->twigVariables !== null) {
+            return $this->twigVariables;
+        }
+
         $frontPage = $this->getConfig('content_dir') . 'index' . $this->getConfig('content_ext');
         return array(
             'config' => $this->getConfig(),