Make it possible to compare PDFs where one or more of the PDFs have cropmarks

[markpenny]

We need to be able to detect cropmarks and ignore them in either/both of the PDFs when comparing so that it doesn't trip up on changes beyond the edge of the page.

Similarly, there needs to be a way to offset the starting page number to start comparing the two PDFs. This will enable PDFs with intro matter before the main body to be compared, or specific/individual Bible books against a larger publication.

[davidg-sil]

This is basically a case of cropping to the crop-marks or masking out page numbers before the comparison. I assume that the right information is in the PDF for the cropmarks, but we'd need to add something, I presume to flag the page number position. Would that be a special comment (assuming PDFs have comments?), or some kind of additional PDF structure (about which I know nothing useful).