Usually when the PDF file contains incremental updates the updates are
appended at the end of the document. But this is not required, the
various trailers can be in any order. Make sure that we look at the last
trailer (logically last, not the one with the largest file offset) when
looking for pages.

Change-Id: Idcd85a7c6bbf08c9436dd73933d79cdb683f482c
Reviewed-on: https://gerrit.libreoffice.org/36527Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>