Bug 18550: Patron card creator: Print output does not respect layout units

To reproduce:
Text, images and barcode positions are always layed out based on PostScript points, regardless of unit defined in card layout.

To reproduce:
- Test on top of Bug 18541 (introduces layout grid)
- Create a card layout with a text field, an image and a barcode. Use points
  as layout unit
- Activate layout grid
- Create PDF output, save
- Change layout unit to mm
- Create PDF output, save
- Compare PDFs. Verify that the positions are unchanged (still using points)

To test:
- Apply patch
- Create again PDF
- Verify that elements are positionad as expected (using unit, e.g. mm)
- Bonus test: Repeat with other units

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>