Converting to CSSPixels caused us to lose precision and the sign of
signed zeroes.
The values we resolve against in Length::ResolutionContext are still
themselves rounded too early but this is in the right direction.
This ensures that we clamp values for properties like padding-* to valid
ranges (non-negative in this case) if they are specified with `calc()`.
The length-related changes in this commit combined with the ones from
the previous commit fix the primary layout issue on https://lwn.net
(yes, not the first place I would have expected problems either).
Math functions like abs(), clamp(), round(), etc, can be used by
themselves in property values, without wrapping them in calc().
Before this change, we were neglecting to run calc simplification on the
generated calculation node trees. By doing that manually after parsing a
standalone math function, we score at least a couple hundred WPT points.
