Because we store calculations as a tree of CalculationNodes inside a
CalculatedStyleValue, instead of a tree of StyleValues directly, this
implements a create_calculation_node() method on CSSNumericValue.
CSSMathValue::create_an_internal_representation() then calls
create_calculation_node() on itself, and wraps it in a
CalculatedStyleValue.
Lots of WPT passes again! Some regressions, which are expected: `cursor`
fails a test for the same reason it fails other that set it to some
kind of numeric value: We don't distinguish between "can contain a
number" and "can accept a number by itself". This will affect any
similar properties, but overall this is a big improvement.
A lone CSSUnitValue can now be converted to a dimension StyleValue of
the relevant type, as long as the property allows that type. If the
value is out of the allowed range, it's wrapped in calc().
There are a few failing tests still, involving setting a negative
percentage and expecting to read the computed value as 0. Those also
fail in Chromium, and a similar negative-length test expects a negative
computed value (not 0), so this appears to be an incorrect test.
Also, we regress some of the `cursor` tests. This is because our "does
property X accept type Y?" code is too naive: `cursor` is defined to
accept "number [-∞,∞]" in the JSON, and that value range is used when
clamping the result of calculations or interpolation. But because that
entry is there, we think a single number is a valid value for `cursor`.
Solving this generally is a larger task than I want to take on right
now. :^)
