This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
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.
In particular, we now blockify layout internal boxes (e.g table parts)
by turning them into `block flow`. This fixes a crash when viewing
our GitHub repo :^)
