Add support for IFT and IFTX tables

[wmedrano]

This is an upcoming font format described at https://www.w3.org/TR/IFT/#font-format-extensions

• ttx support would be good for debugging

[wmedrano]

FYI: I'm working on this. Is there any preference between using otData vs a custom implementation?

The weirdest part of the IFT spec is the use of a union type

Type	Field	Description
uint8 / uint16	entryIndex[glyphCount - firstMappedGlyph]	The entry index for glyph $i$ is stored in entryIndex[i - firstMappedGlyph]. Array members are uint8 if maxEntryIndex is less than 256; otherwise, they are uint16.

[behdad]

otData is highly preferred.

For the union type you mention, I think a custom type makes sense. Just keep them minimal to what's absolutely necessary.

You can even use otData to declare two members: entryIndex8 and entryIndex16 and conditionalize one on mapEntryIndex < 256 and the other on mapEntryIndex >= 256, then add a postRead method to combine them. Likewise, use preWrite to convert back.

[wmedrano]

Current Status

• Format2 supported for IFT and IFTX

Next Steps

• Play around with it and monitor for any issues
• Implement Format1
  • Low priority (I'm not actively working on it) as we don't have any Format1 fonts yet.