Couldn't merge the fonts, inconsistent GDEF.table.MarkGlyphSetsDef

originally reported by @rsms at https://github.com/googlefonts/fontmake/issues/894#issuecomment-1699524088, filing as separate issue as reminder to fix and as well as aid for others who may stumble on it.

The error is triggered by the following change in ufo2ft kernFeatureWriter https://github.com/googlefonts/ufo2ft/pull/720

@simoncozens suggested a workaround

> The simple solution is to zero the width of the commabelowcomb in all masters. Currently they're treated as zero-width marks in some masters but not in others.

Start by reviewing the linked ufo2ft PR 720 and the fontmake compilation path that triggers the inconsistent GDEF.table.MarkGlyphSetsDef error. Reproduce the failure with masters where commabelowcomb has inconsistent widths, then verify that compilation completes without the merge error after the underlying inconsistency is addressed.

Written by the indexing model from the issue text.