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This refactor enables two very significant improvements to our font
handling, which I will be implementing next:
1. Automatically adjust size of fallback faces to better align with the
primary face, a la CSS
[`font-size-adjust`](https://developer.mozilla.org/en-US/docs/Web/CSS/font-size-adjust).
[^1]
2. Move glyph resizing/positioning out of GPU-land and in to
`renderGlyph` and apply alignment/resizing rules from the nerd fonts
patcher[^2] to the glyphs at rasterization time, so that we can ensure
exact cell fits and swap out our embedded JB Mono with an unpatched
version with a separate dedicated symbols-only nerd font.
In addition to being necessary prep work for those two changes, this PR
is also a minor but real stand-alone improvement. By only computing the
cell metrics for our primary font, we avoid a *lot* of wasted work when
loading fallback fonts, and also avoid that being a source of load
errors, which we don't yet handle gracefully[^3].
To validate this PR I've run the full set of font backend tests locally
on my Mac with no failures, and did a sanity check of running Ghostty
with both renderers and with CoreText and FreeType font backends and
then `cat`ing a file that requires fallback fonts to render, and
everything looks correct.
[^1]: #3029
[^2]: The alignment and resizing rules for the nerd font symbols are
defined in the patcher
[here](6d0b8ba05a/font-patcher (L866-L1151))
[^3]: #2991