We have this code duplicated in multiple places, and we'll want to
handle registered custom properties too at some point, so wrap it in a
reusable `CalculationContext::for_property()` method.
Noticed while doing this that ValueParsingContext will eventually need
to take a PropertyNameAndID, not a PropertyID, so I've added a FIXME.
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. :^)
