difference in handling of metric values in some glyphs sources

This comes up with 'underlineThickness', in Playfair.glyphs.

What is particularly interesting in this case is that the diff goes away if we first convert from glyphs to designspace, and _then_ compile, so it seems like possibly some interaction between fontmake/ufo2ft and glyphslib?

Repro:

```
python3 -m ttx_diff 'https://github.com/googlefonts/Playfair?a49f9f9dc9#sources/Playfair.glyphspackage'
```

Start by running the provided ttx_diff command against the Playfair glyphs source and inspect the underlineThickness difference. Compare the direct glyphs compilation with the glyphs-to-designspace-then-compile path, focusing on the reported interaction between fontmake/ufo2ft and glyphslib. Done means identifying and correcting the differing metric handling, with the reproduction no longer showing a diff.

Written by the indexing model from the issue text.