Record when computed properties depend on viewport metrics while
resolving lengths. Carry that information through font metrics so
font-relative lengths can be associated with viewport-sized fonts.
This keeps the dependency tracking local to style computation and gives
later viewport resize invalidation a way to find affected elements.
The computation context used is the main thing distinguishing the
computation of font/non-font properties so having a generic method to
handle this will allow us to consolidate logic between the two.
These get computed to an equivalent RGBColorStyleValue.
To support this, we now store the computed color-scheme on the
ComputationContext when computing properties that might contain a color.
This has a nice bonus of correcting the css-accent-color test's result.
Passing the `AbstractElement` rather than the
`TreeCountingFunctionResolutionContext` allows us to only compute the
resolution context when necessary (i.e. when we actually need to resolve
a tree counting function)
Tree counting functions should be resolved at style computation time -
to do this we will need to know the element's sibling count and index.
This commit computes that information and propagates it to the various
`StyleValue::to_computed_value` methods.
This struct will in the future hold information other than a length
resolution context (e.g. context for tree counting functions) and a
single struct is easier to work with than multiple parameters.
