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.
This solves an awkward dependency cycle, where CalculatedStyleValue
needs the definition of Percentage, but including that would also pull
in PercentageOr, which in turn needs CalculatedStyleValue.
Many places that previously included StyleValue.h no longer need to. :^)
