corner components can differ in some cases

Or at least in one case. Have not investigated further.

```
python3 -m ttx_diff 'https://github.com/googlefonts/Libre-Franklin?310b7eef1f#sources/LibreFranklin-Italic.glyphs'
```

Start by reproducing the report with `python3 -m ttx_diff` and the linked Libre-Franklin `LibreFranklin-Italic.glyphs` source. Compare the corner components produced for the reported case and trace the fontc path that handles them. Done means determining the cause and making corner components consistent in this case, with a regression check if the project provides an appropriate location.

Written by the indexing model from the issue text.