QWidgetPrivate::setRenderToTexture breaks QMainWindow::setUnifiedTitleAndToolBarOnMac

This means that setting this flag and adding a QQuickWidget, QOpenGLWidget or QWebEngineView as a child of the main window will show the toolbar as black (the clear color used by QPlatformBackingStore::composeAndFlush).

I didn't find a way to get glClear in composeAndFlush not to overwrite the gradient left by the NSTexturedBackgroundWindowMask flag.

QMacStyle will test this flag and then paint the toolbars and tabs background as transparent in this case. If it is not possible to support we should make sure that the style paints normally when QWidgetPrivate::textureChildSeen is set on the QMainWindow.