As far as I can say, this reusing is just an optimization, but
it breaks e.g. with <w:pict><v:group><w:pict>... , where
using the same context for two shapes breaks things, e.g.
oox::vml::ShapeContainer::getFirstShape() does not work right.

Change-Id: I90bb85b4d320e1bb0727401879b0551abbac5f12