Bracket layer generation: Support "Rename Glyphs" custom parameter

One of our projects is using Designspace rules to achieve a "Rename Glyphs" like effect, but for a VF. Roundtripping deletes them because glyphsLib discards the information. It might be feasible to store them as a "Rename Glyphs" custom parameter in some master or something? This might be a whole new can of worms though.

Start by tracing how glyphsLib roundtrips Designspace rules and represents Glyphs custom parameters. Determine where the "Rename Glyphs" information is discarded and whether storing it on a master is viable; done means roundtripping preserves the parameter and the resulting variable font retains the intended glyph renaming behavior.

Written by the indexing model from the issue text.