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
Change how we store type of columns. It was used where the specification
only distinguishes between percent and everything else, so it makes more
sense to store and use it as a boolean.