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.
We'll want to remove the LibGUI dependency from the WebContent process.
This is the first basic step of removing unnecessary LibGUI includes
and swapping out GUI::Painter for Gfx::Painter.
