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.
Using fixed-point saturated arithmetics for CSSPixels allows to avoid
accumulating floating-point errors.
This implementation is not complete yet: currently saturated
arithmetics implemented only for addition. But it is enough to not
regress any of layout tests we have :)
See https://github.com/SerenityOS/serenity/issues/18566
The specification says we should distribute excess width proportionally
to the width of the cell, not to the preferred increment. Doing the
latter leads to distributing all excess width to just the cells which
demand some increment, even if it's very modest. Moreover, there's code
which partially implements the correct criteria just below the one we
remove here.
