So that large images are written only once to the ZIP container when
they are exported using both markups. This affects drawinglayer images,
the Writer ones are handled directly in sw and were already
deduplicated.

(cherry picked from commit b484e981)

Conflicts:
	oox/source/export/drawingml.cxx

Change-Id: Iff7c769329b42939833056b727b070f6a60da5e3
Reviewed-on: https://gerrit.libreoffice.org/18491Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>