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.
