font.interpolate with incompatible layerOrders + a public.default layer name

[typemytype]

When two sources have different defaultLayers and different layerOrders and one of the layers is called public.default, it is not possible to write to disk as public.default must be the default layer (see the UFO spec)

Some options:

• rename the public.default layer name if this is not the default layer
• only apply interpolation on the defaultLayer (whatever the name is) font.interpolate also means interpolating all layers, so I dont like this option
• only interpolate layers with the same name (my preference, as you can always do layer.interpolate or glyph.interpolate) + warn missing layer names. This should not break existing implementations.

came up in https://robofontcommunity.slack.com/archives/C03A6AYH6TE/p1667999235159179

[typemytype]

a quick fix as fontParts already does the 3th option: only interpolate layers with the same name (see)

dont set the font.defaultLayerName based on the layerOrder, but check first if "public.default" is available as layer name