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
The used width is already a content width, which doesn't include
borders. Border widths should be subtracted from the specified width
instead, since that initially specifies the total width including
borders, for consistent comparison. Also handle table box padding as an
additional fix.
Adding undistributable space right before setting the content width is
incorrect when it's a percentage. Follow the specification and add it to
GRIDMIN and GRIDMAX instead.
