io-svg.c: first attempt at handling content after an XInclude

io-svg.c: first attempt at handling content after an XInclude

It turns out that gtk3 (but not gtk4) will sometimes wrap its
<xi:include> tag with a <g> tag that specifies some opacity; see
gtk/gtkicontheme.c in the gtk+-3.0 sources. Currently we are keeping
everything before the <xi:include> tag and dropping everything after
it, which results in mismatched tags because we keep the <g> and then
chop off the </g>. I've hacked something together that looks like it
works but it needs a post-sleep reread.