Instead of just measuring the layout viewport, we now measure overflow
in every box that is a scroll container.
This has the side effect of no longer creating paintables for layout
boxes that didn't participate in layout. (For example, empty/anonymous
boxes that were ignored by flex itemization.)
Such boxes are now marked as "(not painted)" in the layout tree dumps,
as they have no paintable to dump geometry from.
Since there are no table-specific boxes anymore it would be nice to
output their types additionally in layout dump so we can tell table
boxes from "regular" boxes.
Solves conflict in layout tree "type system" when elements <label> (or
<button>) can't have `display: table` because Box can't be
Layout::Label (or Layout::ButtonBox) and Layout::TableBox at the same
time.
