.glyphs sources converted from DS+UFO with glyphsLib may contain ufo2ft-specific filters in userData

which fontmake happily applies, whereas fontc doesn't know about

E.g.

```
python3 resources/scripts/ttx_diff.py 'https://github.com/NaN-xyz/Rubik-Filtered?c89f25696f#glitchpop/sources/RubikGlitchPop.glyphs'
```

Will show a bunch of diffs in the component names because it actually runs the `flattenComponents` filter from ufo2ft, because it is defined inside the Glyphs' masters's `userData` (the equivalent of the `lib.plist` from UFO-land, a bag of residual private app-specific stuff that's carried over, often without the font developer even knowing or remembering...)

fontc should probably look for these when compiling from .glyphs inputs.