This fixes an issue where only the last KeyframeEffect applied to an
element would actually have an effect on the computed properties.
It was particularly noticeable when animating a shorthand property like
border-width, since only one of the border edges would have its width
actually animate.
By deferring the invalidation until all animations have been processed,
we also reduce the amount of work that gets done on pages with many
animations/transitions per element. Discord is very fond of this for
example.
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
