Fix argument orders for Ghostscript
This fixes two errors in commit 017927b4 ("Unify calling convention for command-line and API GS use"):
- In postscript->pdf (scm/backend-library.scm), the /PageSize was set before starting the dictionary with 'mark'. This is a lucky coincidence that it didn't throw, but as a result all PDFs were using US Letter format instead of A4.
- In make-ps-images (scm/ps-to-png.scm), the command line arguments didn't match the order in postscript->pdf. This resulted in continuous restarts of the API interpreter whenever switching from PDF to PNG output or back, leading to largely increased CPU time for 'make doc'.