oox: fix handling of noop parent transformation from DOCX

It seems that in Word, a zero parent transformation, like:

<a:chOff x="0" y="0"/>
<a:chExt cx="0" cy="0"/>

means the child shapes will have an absolute position / size. We
actually use this feature in the exporter, but so far the importer
didn't handle this.

Change-Id: I250784a3dddd23649e391b548fc128dfdf153614