The oox::drawingml::Shape -> css::drawing::XShape converter doesn't
support ZOrder, so just give each drawingml::Shape a default ZOrder.
This way the offsets can be applied, and sorting can move the shapes to
their correct place.

This makes parent text of the bugdoc readable.

Change-Id: Ib87a096fba66aad4a4f35d19473ea88dab340fd0
Reviewed-on: https://gerrit.libreoffice.org/63478Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins