LibWeb: Don't traverse entire layout tree in every HTML::EventLoop tick

Instead, collect a list of all the elements with content-visibility:auto
after layout.

This way we can skip the tree traversal when updating the rendering.

This was previously eating up ~300 µs of the 60fps frame budget on
our GitHub repo pages (and even more on large pages).

Libraries/LibWeb/Painting/ViewportPaintable.cpp

@@ -395,6 +395,7 @@ void ViewportPaintable::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(clip_state);
+    visitor.visit(m_paintable_boxes_with_auto_content_visibility);
 }
 
 }