DOCX export: fix image counters for multiple documents
DOCX export: fix image counters for multiple documents

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

Change-Id: Ia4ed1bb5095cc1ade78a404ec3328b7d1349dc3d

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core