ParsedFontFace and FontLoader now both keep track of which
CSSStyleSheet (if any) was the source of the font-face, so the URLs can
be completed correctly.
To prepare for introducing a CSS::URL type, we need to qualify any use
of LibURL as `::URL::foo` instead of `URL::foo` so the compiler doesn't
get confused.
Many of these uses will be replaced, but I don't want to mix this in
with what will likely already be a large change.
