scanSvgRefs
Returns the paths of the external documents content references through <use>.
One cheap pull-parse pass collecting every <use> element's href up to any # — the fetch pass for parseSvg's templates map: scan the layout, fetch each referenced document, re-scan those for transitive references, then parse once with the full map. Nothing is validated here; a malformed <use> is the parse's diagnosis to make.
Return
The referenced document paths, deduplicated; empty when the document has no <use>.
Parameters
content
The SVG document text.
Throws
XmlException
when the text is not well-formed XML.