Support ISO 15924 script tag?

[dhardy]

Swash supports converting a Script to an OpenType tag.

Many (most?) libraries dealing with fonts and shaping appear to use ISO 15924 instead, so also supporting conversion to this tag type would be useful for compatibility.

The two are broadly compatible (it seems like one could just subtract 32 << 24 from the OpenType tag to convert in most cases), but with a few differences (e.g. Tols is ISO 15924 only; dev2 and a few other "v2" tags are OpenType only).

[dhardy]

Use-cases:

• Replace this Parley fn
• Convert to fontique Script for font-face selection without Parley
• For shaping using Harfbuzz / rustybuzz

[memononen]

Another thing to note about OT tags is that both hiragana and katakana map to kana, while in ISO 15924 they have own tags Hira and Kana. So the mapping is not 1:1 in all directions.