This fixes tdf112618, which wasn't importing properly anymore
since recent commit cc899c69
for bug 101826.
I tried to setFly( XFillTransparencyItem(100) ), but then
it didn't round-trip. Fixing up the export sounds like a
suicide mission, so I just used the non-regressive path.

There is still a chance that unset transparency could cause problems
with other types like solid/gradient/bitmap.  However, I wasn't
able to design any example documents to prove that. One difference
is that the RES_BACKGROUND looks in the parents (bInP) for
attributes, while I only transfer directly set attributes(!bInP).

Change-Id: I5160562e222629510ede3b49ae85ec99f7b351c9
Reviewed-on: https://gerrit.libreoffice.org/71646
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>