svg: improve closed-by-style (path and polyline)

Handle one more SVG variation to make path-variations.svg get
it's first blue box (instead of transparent).
svg: comment on <use/> styling and a leak fix
[embedded image] reflect data attribute in dtd
svg: remove second setting of xmlns
Bug 707497 - Fix fatal PNG error wit libpng16

Thanks to Evgeny Bobkin for creating the fixed image.
https://bugzilla.gnome.org/show_bug.cgi?id=707497#c9