Builder: Petition for shrinking instances to GF specs

Catch 22 situation every time:

Font has several variable axes, say `wght` and `slnt`.

Either I need to generate instances that aren't supported by GF (Regular Slant Left) in order for the Slant axis to even be included at all (correct `STAT` table), but then the instances aren't supported in the end, or I deactivate the instances and then the axis isn't getting included and I have to manually create a `STAT` table.

It should be trivial to shrink the instances to the ones supported by Google Fonts (when using the appropriate GF flag).

Please consider to include this. It's a very common problem.