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This lead to QQuickWidget grabbed content to be outdated (render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>

Author: pasnox

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);
         }