Account for dimensions of background pdf

[florian-wagner]

I originally thought it was due to cropping on the reMarkable, but the dimensions of the device simply differ from DIN A4 documents, such that notes on such documents are currently shifted. This PR takes into account the original pdf size and adds a corresponding margin to the right hand side, such that background.pdf and foreground.pdf have the same dimensions and notes are properly aligned.

Note: This is not affected by cropping, yet currently only tested with Din A4 portrait documents

Example

• Original background PDF (DIN A4 dimensions)
• Export with current master
• Export after this pull request

Additional minor changes

• Introduce symlinks instead of copying $ID.pdf to background.pdf, which is more performant for large pdfs (i.e. books).
• Due to the change from pdftk arguments multibackground to multistamp (which accounts for transparency), the foreground and background were flipped (hardly noticeable due to transparent templates). This is also fixed in this PR.

[florian-wagner]

P.S.: This would close #11

[florian-wagner]

There was a bug for size correction in multipage PDFs, which has been fixed by https://github.com/reHackable/maxio/pull/12/commits/7ae588580500f0c68716796101feb345fd54c0a0