Docx writer:  Make images work in reference.docx headers/footers.

* All media from reference.docx are copied into result.
* Added defaults for common image types to [Content Types].
* Avoided redundant XML parse + write for entries taken over from
  reference.docx, for better performance.
Docx writer:  Improved entryFromArchive to avoid parse.

No need to parse the XML if we're just going to render it
right away!
Docx writer: Create overrides per-image for media/ in ref docx.

This should be somewhat more robust and cover more types
of images.