Repaint QOpenGLWidget when screen changes

Commit 5c7f000cd4 greatly reduces the
number of repaints for QOpenGLWidget. Unfortunately, this included when
the widget changes screens.

Change-Id: Iaabcb94925e4519cb5d8561b47aaddcfdc7b01ac
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/widgets/kernel/qwidget.cpp

@@ -9146,6 +9146,9 @@ bool QWidget::event(QEvent *event)
             const QWindow *win = te->window;
             d->setWinId((win && win->handle()) ? win->handle()->winId() : 0);
         }
+#ifndef QT_NO_OPENGL
+        d->renderToTextureReallyDirty = 1;
+#endif
         break;
 #ifndef QT_NO_PROPERTIES
     case QEvent::DynamicPropertyChange: {