Implement anchor propagation for composite glyphs

[rsheeter]

@anthrotype reports there is a fiddly process by which composite glyphs acquire anchors from their components.

• Glyphs has intelligence here; glyphsLib tries to match it but probably doesn't completely do so
  • https://github.com/googlefonts/glyphsLib/blob/main/Lib/glyphsLib/builder/anchor_propagation.py
  • issues with composites' anchor propagation glyphsLib#368
  • How to have mark/m2mk features from Glyphs source? fontmake#682 has the actual Objective-C and a python port
    • implementing this may produce different results than the current glyphsLib implementation; it might be necessary to update both
    • there are also references to the Glyphs logic changing; ask Georg :)
• ufo2ft has it's own implementation and they may not match
  • https://github.com/googlefonts/ufo2ft/blob/main/Lib/ufo2ft/filters/propagateAnchors.py
  • it is not completely clear why this is true

It is not clear to what extent font developers rely on this functionality.

[rsheeter]

Porting the python implementation would be a good start.