benchmark PDFium and poppler; compare the footprint

As with QTBUG-77517: if we are going to tell customers that they should use QtPDF, and the rendering is based only on PDFium, that should be based on something more than just the license.

In particular we should figure out the cost in both space and time. We know that using PDFium implies bundling a rendering engine (Skia) and probably a JS engine (V8?) But those are already built as part of qtwebengine anyway: so we can say that Qt already bundles those engines if you are using qtwebengine.

So which way is faster?

If we do something for QTBUG-77505, the footprint is meant to shrink. But will performance improve or get worse?