Core: fix "Save Image" giving dark area as compare to viewport

Signed-off-by: Yash Suthar <yashsuthar983@gmail.com>

src/Gui/View3DInventorViewer.cpp

@@ -1776,6 +1776,8 @@ void View3DInventorViewer::savePicture(int width, int height, int sample, const
     }
 
     root->addChild(getHeadlight());
+    root->addChild(getBacklight());
+    root->addChild(getFillLight());
     root->addChild(camera);
     auto gl = new SoCallback;
     gl->setCallback(setGLWidgetCB, this->getGLWidget());