Writer and RTF is 1-based, OOXML is 0-based. So it means we do an offset
roundtrip during import, and no offsets are needed on export.

Change-Id: I52cf65f268940d1d43ae76e58d0c7e6de3a54073
Reviewed-on: https://gerrit.libreoffice.org/65827
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>