`com.schriftgestaltung.Glyphs.originalKerningGroups` impacts compilation of mixed kerning rules

So this is a bit of a funny one?

I'm looking at [Solitreo.glyphs](https://github.com/ladinoprojects/solitreo?9b03239708#sources/Solitreo.glyphs) which has a very minor compilation difference in the generated kerning lookups.

Digging into this, it seems to be related to the presence of both LTR and RTL kerns, along with the `com.schriftgestaltung.Glyphs.originalKerningGroups` key.

Looking at `glyphsLib`, the presence the `originalKerningGroups` key in userData impacts how the groups.plist is generated.

This seems simple enough to match, but I do want to make sure that I'm understanding the logic:

- is this key only present in a glyphs file that has been converted from a ufo by glyphsLib?
- is having this impact the compilation of glyphs sources intentional?

below is a minimal test case, if ever anyone else is curious about this:

[Solitreo-kern.glyphs.zip](https://github.com/user-attachments/files/23344366/Solitreo-kern.glyphs.zip)

Start with the attached Solitreo-kern.glyphs minimal case and trace how glyphsLib handles com.schriftgestaltung.Glyphs.originalKerningGroups when generating groups.plist. Compare the resulting kerning lookups for mixed LTR and RTL rules. Done means establishing whether this key should affect compilation and documenting or reproducing the intended behavior.

Written by the indexing model from the issue text.