Add support for variable COLR tables using VarIndexBase and DeltaSetIndexMap

[anthrotype]

Towards #53

Requires the https://github.com/fonttools/fonttools/compare/wip-variable-colr?expand=1 branch of fonttools to work

EDIT: the branch is now called https://github.com/fonttools/fonttools/compare/variable-colr?expand=1 (without the "wip-")

[anthrotype]

oops, clicked the green button too soon 😅
well, i'll mark it as a draft for it requires stuff that hasn't been merged in fonttools yet

[justvanrossum]

Thanks for getting this started! Much apreciated.

[anthrotype]

I suppose this PR needs tests to be added. I am not familiar with the test suite. Mind leaving that to you? I'll be on leave for a couple weeks from tomorrow.

[justvanrossum]

Thanks! Are there any Variable COLRv1 test fonts around?

[justvanrossum]

I suppose this PR needs tests to be added. I am not familiar with the test suite. Mind leaving that to you?

Absolutely. Thank you so much for the implementation!

[anthrotype]

if you want a really simple one you can cd in fontTools/Tests/varLib/data and run fonttools varLib TestVariableCOLR.designspace and you get a simple TestVariableCOLR-VF.ttf with two color glyphs glyph, A and B that do very simple things.
@drott is working on more test fonts in googlefonts/color-fonts#102

[justvanrossum]

I added the fonttools test font + glyphs to the test suite. Can you briefly verify that the images in the expected output are as you expect them to be?

[anthrotype]

Can you briefly verify that the images in the expected output are as you expect them to be?

yes, they look correct, thanks!

[justvanrossum]

Fantastic! More test cases would be nice, but let's do that later.