Improve WriteCustomGeometry in oox export; review of patch

[Regina Henschel <rb.henschel@xxxxxxxxxxx>]

Hi Miklos, hi all,

the patch in https://gerrit.libreoffice.org/c/core/+/131837 is now ready 
for review.

It is a change in the way a custGeom element is generated. I have 
dropped the export via PolyPolygon. Instead our path commands are 
directly mapped to oox path commands. That is a larger change, so it 
would be good, if you find time to review it.

The commit message is quite detailed. If something is still unclear, 
please send me a mail.

BTW, Jenkins fails for a reason I don't know. But I want to first 
incorporate your suggestions before I start a next try with Jenkins.

Kind regards,
Regina