The bug document has:
      <style:page-layout-properties ... style:repeat="no-repeat">
        <style:background-image ... style:repeat="stretch"/>
      </style:page-layout-properties>

When a stretched background image is set on a page style using the Writer UI,
then these two style:repeat attributes always match, but not in the bugdoc.
The later used to have priority, till commit
7d9bb549 (Related: #i124638# Second step of
DrawingLayer FillAttributes..., 2014-06-02).

Fix the problem by extending XMLBackgroundImageContext::EndElement(): if we
know that the <style:page-layout-properties> sets the FillBitmapMode property,
then don't blindly set BackGraphicLocation, but try to overwrite the exiting
FillBitmapMode one.

Change-Id: I64ab4363b20cc95003d35acd63ea421472b1c071
(cherry picked from commit 6621da38)
Reviewed-on: https://gerrit.libreoffice.org/18582Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>