[Glyphs3] support alternate layers

Font doesn't build if has alternate layers.

Easy to go back to format 2 using intermediate layers, but not for alternate ones.
I'll try to find a test case in Google Fonts and link it here.

Start by reproducing the failure with a Glyphs3 source file containing alternate layers; the issue does not identify a file or existing test case. Trace the source-to-UFO conversion path and add a regression case, with completion shown by the alternate-layer font building successfully.

Written by the indexing model from the issue text.