pdfposter: align long single page PDF at top of output page

Jaap Keuterrequested to merge
JaapKeuter/pdfposter:align_long_pdf_to_top into develop

At least this gives the expected result with a long single page PDF of a web page (which needs to be scaled down), and also from the examples/testpage-tall.pdf (which needs to be scaled up).

However there must be numerous other scenario's that may need to be verified first.
I'm unfamiliar with this testing environment, nor have a feel of the coverage provided for this scenario, so I leave that to more knowledgeable people.

Fixes #22

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