Use virtual function to retrieve interface pointer

xfa/fwl/lightwidget/cfwl_picturebox.h

Index: xfa/fwl/lightwidget/cfwl_picturebox.h
diff --git a/xfa/fwl/lightwidget/cfwl_picturebox.h b/xfa/fwl/lightwidget/cfwl_picturebox.h
index ffc67f4446109ea53703e23be1b497ee2f3ecee1..612ff1d84d3a7e843845563d30810b5996dfa3d9 100644
--- a/xfa/fwl/lightwidget/cfwl_picturebox.h
+++ b/xfa/fwl/lightwidget/cfwl_picturebox.h
@@ -16,6 +16,9 @@ class CFWL_PictureBox : public CFWL_Widget {
   CFWL_PictureBox();
   ~CFWL_PictureBox() override;
 
+  IFWL_PictureBox* GetWidget() override;
+  const IFWL_PictureBox* GetWidget() const override;
+
   static CFWL_PictureBox* Create();
 
   FWL_Error Initialize(const CFWL_WidgetProperties* pProperties = nullptr);