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
Now that we have a way to resolve calc() lengths without a layout node,
we can finally support calc() values in font-size.
This wasn't possible before because font-related properties have to be
resolved eagerly in StyleComputer due to font-relative CSS length units
depending on the computed font being known.