Implement anchor propagation for composite glyphs

@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
* https://github.com/googlefonts/glyphsLib/issues/368
* https://github.com/googlefonts/fontmake/issues/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.