We implement this by adding a BlockBox::is_scrollable() helper,
and then ignoring wheel events for non-scrollable boxes.
Thanks to FireFox317 for pointing this out! :^)
This is rather crude, but you can now use the mouse wheel to scroll up
and down in block-level boxes with clipped overflowing content.
There's no limit to how far you can scroll in either direction, since
we don't yet track how much overflow there is. But it's a start. :^)
We now apply a paint-time clip to the padding rect of a BlockBox before
painting its inline-level children. This covers some of the behavior
we want from "overflow: hidden" etc but is far from a complete solution.
The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.
