osg 设置显示窗口大小

    viewer->realize();//需要realize，否则窗口为null
    osgViewer::GraphicsWindow *pWnd = dynamic_cast<osgViewer::GraphicsWindow*>
                                      (viewer->getCamera()->getGraphicsContext());
   if (pWnd) {
       pWnd->setWindowRectangle(20, 20, 600,600);
       pWnd->setWindowDecoration(true);
   }
    viewer->run();