Editor groups write `struct segment` to a file in raw form, but had no
code to enforce that this raw form remained stable over time. Various
changes to `struct segment` have repeatedly changed its internal
structure. Each change created an incompatible dialect of the editor
group file, and all the dialects share the same version number.
According to
```
git log -p -L'/struct segment {/,/};/:common/main/segment.h' HEAD --not 0.58.1-d1x 0.58.1-d2x --
```
`struct segment` changed layout in:
* d1c6b89f17 ("Move dsx::segment -> dcx::segment") [D1 only]
* 596ecbb38d ("Rename segment::value to segment::station_idx") [D1 only]
* 6f10a67c09 ("Move segment::sides to end")
* c53b734abb ("Compute slide segments early") [D2 only]
* 40e90fea22 ("Move Light_subtracted[] to Segments[].light_subtracted")
* a65d774c83 ("Improve packing of struct segment")
* c70c6c98b3 ("Remove obsolete segment::degenerated flag")
Mark editor groups as broken to avoid making the mess worse. If anyone
cares about group support, it needs to be rewritten not to depend on the
internal layout of `struct segment`.