This is really similar to commit
4c2172a3 (tdf#106702 PDF export: fix
missing images from Writer headers/footers, 2018-05-22) just this one is
about the size of the output rectangle for JPG content, while the
previous problem was about the position of them.

Also extract PdfExportTest::exportAndParse() from the last two tests to
avoid duplication.

Change-Id: I9812924d505e9fdaca2a95b4990e7aaa5e44fd7f
Reviewed-on: https://gerrit.libreoffice.org/54773Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>