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
Anonymous wrapper boxes inherit style from their layout tree parent,
and since style data is per-layout-node, we have to manually sync them
from parent to anonymous children when something changes.
This is not very elegant or efficient, so I've left a FIXME about
solving it in a nicer way.
This fixes horizontal dog alignment on https://waffles.dog/ :^)
