The `PDFPageViewBuffer`-code is very important for the correct function of the viewer, but it's currently not tested at all.
While the `PDFPageViewBuffer` is obviously intended to be used with `PDFPageView`-instances, it only accesses a couple of `PDFPageView` properties/methods and consequently it's fairly easy to unit-test this code with dummy-data.
These unit-tests should help improve our confidence in this code, and will also come in handy with other changes that I'm working on (regarding modernizing and re-factoring the `PDFPageViewBuffer`-code).
