Support texture healing in OpenType fonts.

[smcallis]

E.g. the Monaspace font supports "texture healing" where there are alternate glyphs that can be chosen based on the glyphs to the left/right (similar to kerning) that can make a big visual difference:

https://github.com/githubnext/monaspace/blob/main/docs/Texture%20Healing.md

[smcallis]

For super extra bonus points supporting interpolating along the axes in variable fonts would impress me too =D