Fix problem where axis with no mappings was erroring out during roundtrip

[tomarcher101]

Problem: during to_designspace execution in the to_glyphs, to_designspace roundtrip, an error occurs when a custom axis contains no mappings.

ValueError: min() arg is an empty sequence

This occurs because Glyphs.app does not store an axis' minimum, maximum and default values, and therefore these values are wiped during to_glyphs conversion. Normally, to_designspace recalculates these values using the sources and the axis' mapping values. However if there are no mappings in the axis, to_designspace will error.

This error can be avoided by adding identity mappings to the designspace, as to_glyphs will keep these mappings, which then provides to_designspace with the required mappings it needs to calculate the minimum and maximum values of the custom axis. However, during runtime to_designspace deletes all identity mappings, thus causing the roundtrip to fail.

The fix is to make to_glyphs create an identity mapping if one is needed (when the are no axis mappings provided in the designspace) using the axis' minimum and maximum values, so that this information is not lost. This identity mapping can then be used to recalculate the min, max values on the next to_designspace trip (which still deletes the identity mappings). Thus allowing roundtrips to not lose information.

Summary:

• to_glyphs now stores axis min, max information as identity mappings to stop the loss of this information which previously caused errors during roundtrip.

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[belluzj]

@tomarcher101 Try to remove the merge commit from master in the Dalton Maag repository, I think that's the one causing the Google CLA problem.

[belluzj]

LGTM (I reviewed this with Tom already). @anthrotype or @m4rc1e what do you think?