Names shouldn’t be required for intermediate/brace layers either

I programmatically created some intermediate/brace layers (without GSLayer.name) in Glyphs, and they worked fine, but glyphsLib isn’t able to export them to UFO layers because of this validation:

https://github.com/googlefonts/glyphsLib/blob/e2ebf5b517d59bec0c9437da3a748c58f2999911/Lib/glyphsLib/builder/builders.py#L297-L304

The condition `not layer._is_bracket_layer()` should be extended to cover any types of layer that have a synthesized name in the built UFO, in particular intermediate/brace layers.

Start in Lib/glyphsLib/builder/builders.py at the validation around lines 297–304. Trace how intermediate and brace layers receive synthesized names in the built UFO, then adjust the validation so unnamed layers that are handled that way can be exported. Done means programmatically created unnamed intermediate and brace layers export successfully to UFO layers.

Written by the indexing model from the issue text.