Support PDF generation/compression without hOCR files

[MerlijnWajer]

This should be a no-brainer, but we need to deal with a few things:

• We use hOCR files to estimate the page size based on the DPI encoded in the hOCR files (if present), otherwise we estimate it.
• The code that generates the initial PDF with text layer obviously relies on hOCR. We could just make a PDF with empty pages of the right size as alternative when we have no hOCR.