RaP: Scale to a square when generating slide images for pres. format

This improves the quality of portrait documents, before they were
1200px when landscape documents got 1600px.

Switching to scaling to a square means that we can use the "-scale-to"
option on pdftocaio, which means that it generates images directly
at the desired size. This can save quite a bit of time (and memory)
if a document was uploaded with extremely large page size.