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
1. Propagate calc() values from StyleProperties to ComputedValues.
2. Actually resolve calc() values when determining the used flex basis.
This makes the "support" section on https://shopify.com/ show up
correctly as a 2x2 grid (instead of 1x4). :^)
