Axis mappings completely upside down

Let's take [Nunito.glyphs](https://github.com/googlefonts/nunito/blob/main/sources/Nunito.glyphs) which is a Glyphs 2 file and convert it to designspace. The weight axis mappings look like this:

```xml











```

In Glyphs, I open the file, change it to file format 3 and save it. When run through glyphs2ufo, the axis mapping now looks like this:

```xml











```

I think this is because Glyphs changed its understanding of the Axis Mapping custom parameter, so that's a pain but it probably can't be helped.

Let's get rid of the Axis Mapping custom parameter, save the Glyphs file, and try again, relying on the axis coordinates in the instances to provide the mapping.

```xml












```

Where did that "1" come from?