- Ensure that width and height attributes don't get specified
twice is both the img tag and the svg include them.
- Omit unnecessary attributes xmlns, xmlns:xlink, and version on
svg element.
- Use 20 character rather than 40 character hashes for generated IDs.
Closes#8965.
for SVG images in HTML5. Closes#8948.
Note that SelfContained does not have access to the writer
name, so we check for HTML5 by determining whether the document
starts with `<DOCTYPE! html>`. This means that inline SVG
won't be used when generating document fragments.
An API change could be contemplated to give more flexibility,
but this is okay for now.
John MacFarlane