Print QML scenes to PDF with the built-in software backend

The now-integrated 2D Renderer in Qt 5.8 has everything needed internally to render onto a QPaintDevice.

This could present an opportunity to implement printing or at least printing to PDF for QML scenes. Unlike the currently available approach, grabbing the scene as an image, this would allow generating proper PDFs.

It is not entirely clear what this would mean API-wise, but is definitely something worth investigating. At first glance some additional public APIs in QQuickWidget to allow redirecting the output to a custom QPaintDevice would likely be sufficient.