Actually trigger the begin_frame event

The end_frame event is already triggered at the end of the render() method.

Author: Mellthas

src/Renderer.php

@@ -58,6 +58,8 @@ public function render(Frame $frame)
     {
         global $_dompdf_debug;
 
+        $this->_check_callbacks("begin_frame", $frame);
+
         if ($_dompdf_debug) {
             echo $frame;
             flush();