Disallow incremental updating for documents that have hybrid xrefs

Alter pdf_can_be_saved_incrementally to return false when a hybrid xref
file has been opened. This doesn't enforce the behaviour, but will ensure
it for apps that determine their behaviour via this test function.

Once changes have been saved to such a file, it is no longer hybrid and so
further saving steps can use incremental updates.

This change was made because we were previously unable to sign hybrid xref
documents in a way that was satisfactory to Adobe Reader. Adobe Reader
reorganises hybrid xref files, thus invalidating any signature.

0 files changed, 0 insertions, 0 deletions