[Experimental] API for layout-stability of variation axes changes?

[drott]

From an explorative chat between @behdad and @drott:

If we were able to tell whether a variation axis change affects layout or not (important metrics: horizontal or vertical advance width, ascent, descent & glyph variations: rvrn...other?) then there is the potential to skip layout and animate only the glyph shape, allowing a rendering engine to skip layout for a variation axis change.

@behdad mentions it's theoretically possible to add API to HarfBuzz to determine whether axes trigger layout-sensitive changes.

Leaving this here to track, if we decide to come back to it.

[behdad]

I agree it probably be very hard for platforms to embrace such API. Leaving it around for a while to see if I can get to it.