Because the writerfilter::ooxml::OOXMLFastContextHandlerShape
constructor only sent the input stream (and not the full media
descriptor) to oox::shape::ShapeContextHandler, it's startFastElement()
tried to reconstruct a media descriptor from just the input stream. As a
result, the base URL of the document got lost at the time the
oox::drawingml::BlipContext constructor tried to call getAbsoluteUrl(),
to convert the relative URL to an absolute one.

Fix the problem by sending not only the input stream, but the full media
descriptor to the shape importer. As a bonus that makes code simpler,
too.

Change-Id: I1f06c04d0745aa7e2e06d4848cf454790d5073ca