Instantiator: rule substitutions behave differently than for OT feature variations

[madig]

Rules that work for variable fonts may not work for instances, because varLib/feature variations and the instantiator work differently.

• Expectation: I can reuse the Designspace with the rules that I made work for variable fonts.
  • instance-test.designspace contains the rules that work for the VF, which I cannot reuse for instances.
• Actually: I have to make a copy and write different rules to implement the same logic for instance generation.
  • instance-reference.designspace contains rules that work. Note how they look different from the rules that I'd like to work for VFs in that I have to start from the original glyphs A and a.

The reproducers are described in more detail on https://github.com/daltonmaag/designspace-rules-reproducers.

What I expect (from instance-ref-stacked-high.ttf, made from in instance-reference.designspace, with the test string "Aa"):

What actually happens (in instance-test-stacked-high.ttf, made from instance-test.designspace):

The "a" is the original glyph rather than "a.break".

The rub lies in the way substitutions work:

1. In feature variations, the shaper is expected to find the first condition set that matches and then stop there.
2. The instantiator will collect all rules where the condition set matches and then swap glyph contents. On rule boundaries (like HGHT=750 for instance-test-stacked-high.ttf), when rules contain duplicate substitutions and start and end points overlap, it may collect two or more rules. Any duplicate substitutions can then cause a swap back and forth, undoing substitutions.