Move CSS animation values into a mutable overlay on computed properties
and make base computed style data immutable after construction. Base
style mutation now goes through a builder that is consumed on publish,
so installed styles no longer expose mutation APIs.
Build new base style data for inherited style updates instead of cloning
and mutating installed computed properties. Element-specific computed
style adjustments now run before publication, while animation and
transition updates continue to mutate only the animated overlay.
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
We currently have 2 virtual methods to inform DOM::Element subclasses
when an attribute has changed, one of which is spec-compliant. This
patch removes the non-compliant variant.
Now that we have RTTI in userspace, we can do away with all this manual
hackery and use dynamic_cast.
We keep the is<T> and downcast<T> helpers since they still provide good
readability improvements. Note that unlike dynamic_cast<T>, downcast<T>
does not fail in a recoverable way, but will assert if the object being
casted is not a T.
