instance.exports is not respected

When an instance has this:
```
exports = 0;
```
it should not be exported when doing `fontmake -g file.glyphs`

Start by reproducing the issue with a Glyphs source containing an instance with `exports = 0`, using `fontmake -g file.glyphs`. Trace how instance export settings are read during conversion and identify the relevant existing tests or fixtures. Done means non-exported instances are omitted from the generated output while exported instances remain available.

Written by the indexing model from the issue text.