Write the native data and refer it from the XHTML stream.

Change-Id: I8952ed07108ddc92b1d136bd9af6f21d3ed604f0
Reviewed-on: https://gerrit.libreoffice.org/50941Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>