[css3-values] Definition of mod in calc()

There are two widely-used definitions of the modulus operator: one
that keeps negative numbers negative, and one that makes them
positive.  As an example, the expression "-8 mod 5" evaluates to
either -3 or 2, depending on which definition you use.  To be safe,
some uses of modulus in pseudocode in our modules writes the math as
"((a mod b) + b) mod b)", which produces the latter answer regardless
of the definition you use.

Which interpretation should the spec use?

~TJ