PaintContext dates back to a time when display lists didn't exist and it
truly represented "paint context". Renaming it to better align with its
current role.
This patch expands our generated content support beyond single strings
to lists of strings and/or images.
Pseudo-elements like ::before and ::after can now use content:url(...)
to insert anonymous image boxes into the layout tree.
This is heavily used in Google Docs for UI elements.
For getComputedStyle(), we must return an absolute URL for image style
values. We currently return the raw parsed URL.
This fixes loading the marker icons on https://usermap.serenityos.org.
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
