For a gradient fill we need to generate a name because
ODP export works with this name.
In case of shapes it works because when fill attribute
changes some internal name generation is triggered.
The same thing doesn't work for slide background
so generate this name explicitely in oox code.

Reviewed-on: https://gerrit.libreoffice.org/33937Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit aeece6f1)

Change-Id: Ic6ebf37ef3d66a9c274747ca04653363b1fe6d02
Reviewed-on: https://gerrit.libreoffice.org/33941Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>