The main change here is that we now properly absolutize values which
means we now support `random()` and `sibling-{count,index}()`
We are also more consistent with how we handle computation for the other
font properties
Previously we implemented an all encompassing `MathDepthStyleValue`
specifically for the `math-depth` property, this was unnecessary since
we can represent `auto-add` and `<integer>` using existing `StyleValue`
classes.
This brings the values created from parsing in line with those set via
`StylePropertyMap` which allows us to simplify computation