macOS bitmap-only fonts are a poorly documented format, which are often
distributed as `.dfont` or `.dfon` files. They use a 'bhed' table in
place of the usual 'head', but the table format is byte-identical, so
enabling the use of bitmap-only fonts only requires us to properly fetch
this table while calculating metrics.
ref: https://fontforge.org/docs/techref/bitmaponlysfnt.html
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