Automatically decompose when 2x2 transforms differ

[simoncozens]

This fixes #507 (again).

• Decompose glyphs if they have non-matching transforms
• Compare the relevant part of the transformation only
• Add a test

[anthrotype]

this is good, thanks Simon!

[anthrotype]

can you also check if #695 gets fixed by this too?

[simoncozens]

I'm not sure. It looks OK to me, but it also looks OK with the main branch as well. However, it was previously giving

WARNING:fontTools.varLib:glyph Ohorntilde has incompatible masters; skipping

and now it doesn't, so maybe?

[anthrotype]

yeah, that's it! varLib is dropping the variations for that composite glyph because the master components' transforms do not match (I think as result of this recent change in fonttools => fonttools/fonttools#2857)

[anthrotype]

if you drop the font built with this branch in fontgoggles and type "ỡỠ" do both vary as you move along the wght axis? /cc @yanone

in that case, I would mark #695 as fixed by this as well

[simoncozens]

Yes, they do. But as I said, they seemed to do that for me before. I think it might involve different versions of fonttools too.