FDO#74430: File Corruption due to repetition of docPr Id

Issue :
 - If there is a scenario where a chart is followed by a shape
   (may it be in the footer or on a different page )
   which is being exported as an alternate content then, the
   docPr Id is being repeated, ECMA 20.4.2.5 says that the
   docPr Id should be unique.
 - Two different variables were been used as counters for the
   docPr Id, hence there were repitions in the docPr Id.

Implementation :
 - ensured that a single counter is used to track the docPr Id.

Conflicts:
	chart2/qa/extras/chart2export.cxx
Reviewed on:
	https://gerrit.libreoffice.org/7916

Change-Id: I1c105be184d045ef058423f03259eb8c634fcbbe