drawingML export: fix position of wrapped shapes in DOCX

In case of DOCX, the DOCX wrapper around the drawingML fragment already
contains the real position of the shape (which talks about paragraphs
and other concepts which are unavailable in a spreadsheet /
presentation), and the inner position should be 0, otherwise the shape
is shifted towards the bottom right corner.

Note that this only affects drawinglayer shapes in DOCX, Writer pictures
are handled in sw, and those always had a 0,0 position in the inner
drawingML fragment.

Change-Id: I582b1ae64387b50ffb051e1f5017eab0e5d5ab34