We now reject fonts where the active cmap subtable is in a format
we can't read yet, instead of silently drawing squares for all glyphs.
This doesn't fire at all for my 1000-file PDF test set, but seems
like a good thing to check.
(Instead of duplicating the switch, I first tried making a
glyph_id_for_code_point_or_else() that returns ErrorOr<u32> and then
make both glyph_id_for_code_point() and validate_format_can_be_read()
call that, but I liked less how that worked out -- felt too clever.)
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