calculation of axes

I just tried to export some of my fonts as variable fonts using fontmake.

At least partially, it tries to determine the axes by looking at the instances. It prints this: `INFO:fontTools.varLib:Axes: ...` that contains numbers from the instances. And if those are not siting at the master location, it gives an error later on:
`assert base_idx is not None, "Base master not found; no master at default location?"`

So the axes should always be computed from the masters (and `Virtual Masters`).

The issue names fontmake and fontTools.varLib but no repository file or test. Start by tracing where fontmake obtains axes from Glyphs instances, masters, and Virtual Masters, then verify that axes come from the masters and that the default master can be found without the reported assertion.

Written by the indexing model from the issue text.