DOCX export: fix image counters for multiple documents

The next commit will have a testcase for this, too.

Change-Id: Ia4ed1bb5095cc1ade78a404ec3328b7d1349dc3d
DOCX export: make sure a graphic is only written once

Even if it's referenced multiple times, because of mc:AlternateContent.

Change-Id: Ie4cb0ec088d12be5421bac43113c8ae4636028e0