A Writer image has an underlying SdrObject, but GetSdrObjectFromXShape()
won't work for it, also we can't an sw SdrObject into an XShape because
SvxDrawPage::CreateShape() has no idea how to handle sw's
SdrInventor::Swg inventor.

Fix the problem by just getting the Graphic of the Writer image and
improve GraphicExportFilter to be able to work with just a Graphic as
well.

Change-Id: I3c9b3005366fcc87815597a27df3cb8a99a8876c
Reviewed-on: https://gerrit.libreoffice.org/54527Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>