Requires a PDF4Qt enabled build. This method exports the QWebEnginePage
to a PDF file, so that we get nicely vectorised objects and
text-as-text. It's the ONLY supported way in Qt API to do this, every
other method of rendering QWebEnginePage forces rasterisation of
the content.
We then use the QgsPdfRenderer class to render the temporary PDF
content to a QPainter device, which keeps vector objects as vectors.
