Add compatibility checker

[simoncozens]

Currently for Radio Canada Italic, fontmake produces this output:

fontmake: Error: In 'sources/RadioCanada-Italic.glyphs' -> 'master_ufo/RadioCanada-Italic.designspace': Generating fonts from Designspace failed: Base master not found.

This is particularly confusing because earlier on in the log it says:

INFO:fontTools.varLib:Index of base master: 4

So why can't it find the base master? Not an easy question for users to answer. With this PR, fontmake says:

ERROR:fontmake.compatibility:Fonts had differing anchors in glyph cedi:
ERROR:fontmake.compatibility: * Radio Canada Condensed Light Italic had "top"
ERROR:fontmake.compatibility: * Radio Canada Condensed Italic, Radio Canada Condensed Bold Italic, Radio Canada Light Italic, Radio Canada Italic, Radio Canada Bold Italic had ""
fontmake: Error: In 'sources/RadioCanada-Italic.glyphs' -> 'master_ufo/RadioCanada-Italic.designspace': Compatibility check failed

The checker is applied when producing variable fonts or whether the Glyphs "Enforce Compatibility Check" custom parameter is turned on.

[anthrotype]

Thanks Simon, this is super useful!! Mind adding a few tests as well?

[simoncozens]

Will do, just as soon as I've got the ones we already have passing...

[simoncozens]

All ready!

[anthrotype]

LGTM, thanks for the patience!

[anthrotype]

maybe squash before you merge

[simoncozens]

Thanks for your patience!

[JeremieHornus]

We have a lot of new issues with this compatibility checker that seem to be bugs in here.
It spots UFO .glif glyphs with "different number of components" where there are actually no components at all :(
please advise

[JeremieHornus]

ah sorry! it was actual compatibility problems in our UFO sources! hard to spot but real issues
so thanks a lot for this 'compatibility check' addition 💯

[simoncozens]

I have a problem with it, though - it's doing compatibility checks on non-exported glyphs, and it doesn't matter if they aren't compatible.

[justvanrossum]

It seems it also complains about anchor order, say "top, bottom" vs. "bottom, top". Shouldn't a case like that "just work"? Or are there cases where the anchor order is meaningful?

[simoncozens]

No, that should just work. I'll sort it.