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
This ensures that min-content contributions from cells with no content
are computed using their calculated values, which are never considered
for min-content before then. The specification diverges from column
measures algorithm, which doesn't use specified width of cells anywhere.