GSGlyph.duplicate(): Thin copying produced wrong results for adding layers to the new glyph.
GSGlyph.duplicate(): Thin copying produced wrong results for adding layers to the new glyph.

Layers were being added to both the old and the new glyph, and
attributes of these added layers wouldn't show up. A deep copy fixed it.

schriftgestalt/GlyphsSDK,schriftgestalt/GlyphsSDK,schriftgestalt/GlyphsSDK