Always retain the default layer

[madig]

Previously, you could drop the default layer if it had been renamed.

Code suggested by @RickyDaMa :)

[RickyDaMa]

Not disagreeing necessarily with Cosimo, but given that currently norad upholds the invariant of the default layer being first, this fix works

[anthrotype]

The correct fix that doesn't depend on any specific layer name or ordering is to treat as default the layer whose path points to the default glyphs directory

[cmyr]

looks good, thanks!