Italic style metadata variable font fixes

Hi, setting `fvar` instance default values for the `ital` axis generally does not work as expected.

In addition, generation of named variation instances for fonts with `ital` (or `slnt`) axis also does not generally work as expected.

Finally, italic angle fixing is unfinished: https://github.com/googlefonts/gftools/blob/v0.9.30/Lib/gftools/fix.py#L417.

I believe the following and other issues might be related: https://github.com/googlefonts/gftools/issues/627.

In general, italic style metadata font fixes appear unfinished and currently need to be done on a case-by-base basis with `fonttools`.

I am partly looking to roll my own fixes for families such as Roboto (currently being served the 2.137; 2017 static-only version) and normalize their metadata to be consistent with the GF spec.

This would also allow packagers and others to easily package or distribute upgraded families while keeping metadata consistent with GF.