Artistic effects on bitmap images in OOXML files create a backup file
with the original image. These were not being saved into the proper
path for PPTX files because the `/word` directory was hardcoded in the
export code.

The path saved to the relations file was incorrect too because it was
not taking into account that the relations file is saved to a subdir
in PPTX and XLSX documents.

Finally, added a unit test.

Reviewed-on: https://gerrit.libreoffice.org/43338Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit 55641f55)

Change-Id: I3e880ce763dc10dbc86ccf0e00871be5d36f6d88
Reviewed-on: https://gerrit.libreoffice.org/43600Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>