Compilation of .glyphs file fails if custom GDEF table present in features

1. Get Designspace and UFOs from `https://gitlab.gnome.org/GNOME/cantarell-fonts/tree/master/src`
2. `ufo2glyphs Cantarell.designspace`
3. `fontmake -g Cantarell.glyphs`

There is a `GDEF` table present in the feature file; fontmake builds a Designspace out of the .glyphs file at `font_project.py:141`, the function builds a `GDEF` table by default but blows up in `glyphsLib/builder/features.py:96` if a `GDEF` is present already.

Options:
1. Mellow out the glyphsLib assert and log a warning if a `GDEF` exists and skip re-generation?
2. Extend the parameter to be a tri-state (generate unless present, generate, don't generate)?

Reproduce the failure with ufo2glyphs Cantarell.designspace and fontmake -g Cantarell.glyphs, then inspect font_project.py:141 and glyphsLib/builder/features.py:96. Determine the intended handling when a custom GDEF table is already present; done means the Cantarell source compiles without the assertion failure and the chosen behavior is covered by an appropriate test.

Written by the indexing model from the issue text.