Kerning between Common script (`Zyyy`) characters is wrong for RTL text

[khaledhosny]

Kerning between glyphs for Common script (Zyyy) is output in a lookup that assumes these glyphs will always be LTR, but for RTL text they will be laid out in RTL direction, so the kerning will be wrong.

[khaledhosny]

We currently put them in kern_Default lookup and add it to all scripts regardless of the script direction, but for RTL scripts there should be another kern_Default_RTL lookup with the kerning values set up for RTL direction.

[anthrotype]

sounds good to me. Maybe you'd like to work on it? :)

I wonder why we hadn't thought about this solution before.