Problem: CSSPixels::abs() invokes UB for the i32 minimum raw value.
Cause: abs() returned from_raw(::abs(m_value)) — and ::abs() of the i32
minimum isn’t representable as an int.
Fix: Compute the magnitude with saturating_sub(0, raw_value()) for
negative inputs — so the i32 minimum saturates to the maximum.
Instead of operating within the (saturating) CSSPixels constraints,
calculate the expected value using a floating point calculation first
and then create the CSSPixels value.
In general it is not safe to convert any arbitrary floating-point value
to CSSPixels. CSSPixels has a resolution of 0.015625, which for small
values (e.g. scale factors between 0 and 1), can produce bad results
if converted to CSSPixels then scaled back up. In the worst case values
can underflow to zero and produce incorrect results.
After the CSSPixels implementation evolved from a wrapper of double
to a fixed-point saturated math arithmetic implementation, it makes
sense to have separate tests for it.
