do a feature comparison between poppler and pdfium

Poppler is older, and open-source, and has had a Qt binding for a very long time. We are interested in using PDFium mainly because of its license (suitable for both open source and commercial software): however we should check whether that results in the company putting its weight behind a better or worse solution. If it turns out that poppler is better in certain ways, we should be honest enough to say so.

If it turns out that poppler is way better, then maybe we should consider making dual backends for QtPDF: provide the same API for both.