Add a phase that eliminates mixed contour+component glyphs; legal in source but not binary

[rsheeter]

Split IR glyphs with contours and components - observed in Lexend - apart such that a glyph with both migrates its contours into a new glyph then references that glyph as a component.

Update the ACL rigging to accomodate; we need the ability to express the ability to write to any glyph including those we've never seen before because we're creating them.

Fixes #125 by converting glyphs with different component definitions at different positions in designspace to simple glyphs.

Split out from #120 to make review more tractable.

The most interesting changes are in fontir/src/glyph.rs.

[anthrotype]

Split IR glyphs with contours and components

Currently, the expectation from font developers for such mixed contour-component glyphs (based on established font editors' behavior, and followed by fontmake itself) is that they should get decomposed to simple glyphs, but it looks like you went the other way around and decided to upgrade them to full composite glyphs (by pushing the contours to separate glyphs and compositing from those).
This does not mean we have to continue to do so. Besides, it seems you have put a great deal of work on this already. Was your thinking that by doing so it increases the chance of overall size savings? Could/should this sort of optimization be left to a separate (optional) step that applies to all glyphs maybe (e.g. #102)?

[rsheeter]

Tbh I was not aware that the font developer would expect that; we don't have to do it this way if it's important. I was indeed imagining this might contribute to size savings.

[anthrotype]

well yes, I'm quite sure that's what all the font editors do in this situation (decompose mixed glyphs)

[rsheeter]

@anthrotype pointed out this approach consumes additional gids whereas reducing a path + components to a single simple glyph does not.

[rsheeter]

Per IM will update to match behavior of existing compilers. Conveniently that's actually simpler.

[rsheeter]

@anthrotype notes in ufo2ft the recursive function that accomplishes that task is called deepCopyContours

[rsheeter]

Another option might be to keep the behavior but off by default

[rsheeter]

@khaledhosny mentioned FontForge defaults to converting to simple glyphs but also has a script command to do what my initial implementation here does.

[rsheeter]

Currently, the expectation from font developers for such mixed contour-component glyphs (based on established font editors' behavior, and followed by fontmake itself) is that they should get decomposed to simple glyphs

This is now the default behavior but can be flagged off.

[dfrg]

Looks good! Functionality seems solid. Left some suggestions inline for style, readability and comments.