xlink:href does not decode percent encodings

When there are non-ASCII characters in an included image's path, the xlink:href can contain those characters verbatim (due to being an IRI reference), but can also contain them in percent encoded form (which is how Inkscape does it). This leads to errors such as ``` Warning (in usvg::parser::image:110): '../auswahl/jpegs/K%C3%A4rnten/IMG_20250806_095603.jpg' is not a path to an image. ``` for SVG code such as ``` <image width="540.49396" height="405.37045" preserveAspectRatio="none" xlink:href="../auswahl/jpegs/K%C3%A4rnten/IMG_20250806_095603.jpg" id="image474" x="-78.537964" y="1193.3698" /> ``` Please consider applying suitable percent decoding. Note that with file:// URIs, there is no strong definition of whether name components whose semantics differ should be escaped, so when a file name contains a semicolon (";"), it might be present either verbatim or as "%3b"; I think that for those cases, conflating them at dereferencing time is fine.