Fix wrong grabbing behavior

QQuickWidget::grabFrameBuffer() was not polishing its items nor
syncing the scene graph compared to standard QQuickWindow::grabWindow().
This lead to QQuickWidget grabbed content to be outdated (render
the previous frame as a new frame).

Task-number: QTBUG-57596
Change-Id: I94f5e0aa5b096fb9e21259267c0e50473e8ed5bd
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/quick/items/qquickrendercontrol.cpp

@@ -380,6 +380,9 @@ QImage QQuickRenderControl::grab()
 
  if (d->window->rendererInterface()->graphicsApi() == QSGRendererInterface::OpenGL) {
 #if QT_CONFIG(opengl)
+ QQuickWindowPrivate *cd = QQuickWindowPrivate::get(d->window);
+ cd->polishItems();
+ cd->syncSceneGraph();
  render();
  grabContent = qt_gl_read_framebuffer(d->window->size() * d->window->effectiveDevicePixelRatio(), false, false);
 #endif
@@ -394,6 +397,8 @@ QImage QQuickRenderControl::grab()
  QPaintDevice *prevDev = softwareRenderer->currentPaintDevice();
  softwareRenderer->setCurrentPaintDevice(&grabContent);
  softwareRenderer->markDirty();
+ cd->polishItems();
+ cd->syncSceneGraph();
  render();
  softwareRenderer->setCurrentPaintDevice(prevDev);
  }