We used to just dump all the object offsets in the xref of the
incremental update, but Adobe Acrobat doesn't like that, and considers
that a second signature invalidates the first. If we properly only
mention new and changed objects in the xref, then this doesn't happen.

This requires actually parsing incremental updates, the previous code
depended on LO writing not-really-incremental xrefs at the end of
incremental updates.

Change-Id: Icdd73fe0a3eab16f8c5a62f1355edbb49f6e73de
Reviewed-on: https://gerrit.libreoffice.org/30288Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>