Enhance `Font::harfbuzz_font()` to include font variation information
when creating HarfBuzz fonts. This required updating the `Font` struct
to store details about font variations.
I wasn’t aware of this, but it also fixed some visual artifacts with
variable fonts, so big thanks to @Lubrsi for the suggestion!
This patch introduces a per-Gfx::Font cache for harfbuzz text shaping
results. As long as the same OpenType features are used, we now cache
the results of harfbuzz shaping, saving massive amounts of time during
text layout.
As an example, it saves multiple seconds when loading the ECMAScript
specification at <https://tc39.es/ecma262>. Before this change, harfbuzz
shaping was taking up roughly 11% of a profile of loading that page.
The cache brings it down to 1.8%.
Note that the cache currently grows unbounded. I've left a FIXME about
adding some limits.
