This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.
There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.
This element doesn't actually support anything at the moment, but it
still massively speeds up painting performance on Wikipedia! :^)
How? Because we no longer paint SVG <path> elements found inside
<clipPath> elements. SVGClipPathElement::create_layout_node() returns
nullptr which stops the layout tree builder from recursing further into
the subtree, and so the <path> element never gets a layout or paint box.
Mousing over Wikipedia now barely break 50% CPU usage on my machine :^)
