When loading other fonts than the system ones, they would get ID -1, not
the one from their resource ID. Even if you later registered the font
name, the old font entries would still be wrong. Another possibility
would be to update _fontRegistry when registering fonts, but sev was ok
with this way so let's go with it for now.
It does not consider the possibility of ID collisions the way
registerFontName() does, but apparently there shouldn't be much risk of
that.
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