Previously we would resolve font features
(https://drafts.csswg.org/css-fonts-4/#feature-variation-precedence)
per element, while this works for the current subset of the font feature
resolution algorithm that we support, some as yet unimplemented parts
require us to know whether we are resolving against a CSS @font-face
rule, and if so which one (e.g. applying descriptors from the @font-face
rule, deciding which @font-feature-values rules to apply, etc).
To achieve this we store the data required to resolve font features in a
struct and pass that to `FontComputer` which resolves the font features
and stores them with the computed `Font`.
We no longer need to invalidate the font shaping cache when features
change since the features are defined per font (and therefore won't ever
change).
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.
