Remove 'open corners' from glyphs sources

[cmyr]

Glyphs.app sources sometimes have a convention where two line segments that that join at an angle include an extra additional tangent-like line segment, like a little hat. These are removed by default in glyphs sources, and we don't handle that.

I have some confusion about the implementation in glyphsLib, but I think initially it will make sense to just port it directly.

See

• Open Corners allowed in Glyphs source?  fontmake#288 and
• https://github.com/googlefonts/glyphsLib/blob/main/Lib/glyphsLib/filters/eraseOpenCorners.py.