We now don't absolutize the URL during parsing, keeping it as a CSS::URL
object which we then pass to the "fetch an external image for a
stylesheet" algorithm. Our version of this algorithm is a bit ad-hoc,
in order to make use of SharedResourceRequest. To try and reduce
duplication, I've pulled all of fetch_a_style_resource() into a static
function, apart from the "actually do the fetch" step.
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.
Having this hidden away in ImageStyleValue meant that
CSSStyleProperties (and anyone else who holds style values) had to know
exactly which types need visiting. This is a footgun waiting to happen,
so make this a virtual method on CSSStyleValue instead.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
