Also avoid librevenge::RVNGBinaryData::appendBase64Data() for
performance reasons. Times with and without the XMLBase64ImportContext
rework for sw/qa/extras/odfexport/data/embedded-font-props.odt:

- before: 1m32.254s
- after: 0m7.045s

(Need to insvestigate macOS font embedding situation in general, later.)

Change-Id: I5aa56bfbfa8dc64f19c021202a1b87618b4b2775
Reviewed-on: https://gerrit.libreoffice.org/45385Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>