Instance generation: if a Designspace contains uprights and an ital axis, the slantAngle in the italic instances might not get set

We currently sniff out the `slnt` axis to set the italic angle on instances, but the `ital` axis goes unchecked. If a DS contains both uprights and italics with an `ital` axis, the slant angle will probably not be set correctly in the italic instances.

Edit: for context, afaik Word requires an `ital` axis for the italicise button to work.