Previously we wouldn't respect font-variation-settings for fonts matched
through `font_matching_algorithm` (i.e. any fonts which didn't have an
exact loaded match).
The `VectorFont` type was renamed to `Typeface` in d5926a3 but this
member variable was never updated.
The getter for this was also unused so has been removed
Computing the font for an element in `compute_font` is premature since
we are yet to apply animated properties - instead we should compute the
value on the fly (with a cache to avoid unnecessary work) to ensure we
are respecting the latest values
Font computation and loading is distinct enough from style computation
that it makes more sense to have this in it's own class.
This will be useful later when we move the font loading process to
`ComputedProperties` in order to respect animated values.
