CSS-Typed-OM has the following hierarchy:
CSSStyleValue
- CSSNumericValue
- CSSUnitValue
- CSSMathValue
- ...various math functions...
Somewhat unintuitively, numbers are also stored using CSSUnitValue with
`unit = "number"`.
There are no distinct classes for LengthStyleValue, etc in the spec, but
they're convenient for us, so they are implemented as subclasses of
CSSUnitValue, at least for now.
Rather than passing an increasingly-unwieldy number of font parameters
individually to every function that resolves lengths, let's wrap them
up.
This is frustratingly close to being `Gfx::FontPixelMetrics`, but bitmap
fonts cause issues: We choose the closest font to what the CSS
requests, but that might have a wildly different size than what the
page expects, so we have to fudge the numbers.
No behaviour changes.
There were a mix of users between those who want to know if the Length
changed, and those that just want an absolute Length. So, we now have
two methods: Length::absolutize() returns an empty Optional if nothing
changed, and Length::absolutized() always returns a value.
