Instead of resolving some viewport-relative sizes on every paint, we now
do them just once in paint-only property update.
This takes SVGPathPaintable::paint() from 15% to 8% in the profile when
scrolling on https://imdb.com/
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.
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
