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).
While our default font supporting variations is unlikely, this is
nevertheless required for our fallback font to be considered equal to
it's non-default/fallback equivalent (i.e. `font-family: serif`) which
in turn is required for LineBuilder to merge chunks into a single
fragment.
When rendering text, if none of the fonts in the cascade list contain a
glyph for a given code point, we now query Skia's font manager to find
a system font that can render it.
