Round-tripping OpenType tags through BCP 47 losslessly

[dscorbett]

Firefox calls hb_ot_tag_to_language and hb_buffer_set_language to implement font-language-override, which causes a bug: the tag is parsed case-insensitively but CSS says the string is case-sensitive.

I would like to fix this by adding a new private use subtag syntax to HarfBuzz: x-hbot-AABBCCDD, where each pair of letters in the trailing subtag represents a byte of the tag. For example, hb_ot_tag_to_language would convert the tag 'trk ' (which is not the right tag for Turkish) to x-hbot-74726b20, which hb_ot_tags_from_script_and_language would later interpret as 'trk ' instead of normalizing the case to 'TRK '.

@behdad @jfkthame What do you think?

[behdad]

I'm not convinced we need this but am open to it. Suggested format looks good to me.