Fix COLRv1 sweeps modulo and paint clipping

[hybridherbst]

Fixes #240 and some related bugs with glyphs from the test font.

[justvanrossum]

Thanks, looks great!

I left one question inline.

[justvanrossum]

Out of scope for this issue, but I've noticed that rendering many adjacent triangles gives some artifacts, and causes some pixels "between" triangles to be more transparent, resulting in a lighter color:

(Top Chrome, bottom Fontgoggles)

This is unlikely something that you have introduced, but I'm curious what your thoughts are about this.

[hybridherbst]

Turns out there's a coregraphics API, CGContextDrawConicGradient, that can render these gradients with less of the triangle line artifacts.
I added a small test font with two glyphs for testing as well, because there was no coverage for "transparent sweep gradients that overlap each other" – so that should be covered now as well.

Can you check if that fixes the transparency issue you see in FontGoggles? I wasn't able to reproduce that.

[justvanrossum]

CGContextDrawConicGradient

Great find, thanks!

This probably means that the fallback _drawPathSweepGradientWithPatches() is no longer tested, right?

Can you check if that fixes the transparency issue you see in FontGoggles? I wasn't able to reproduce that.

It's just perfect now:

Thanks!

[hybridherbst]

Great to hear!

The fallback would to the best of my understanding still be needed for MacOS < 14; if that's not an intended target anymore then it can go. (for reference, https://en.wikipedia.org/wiki/MacOS_Ventura, https://developer.apple.com/documentation/coregraphics/cgcontextdrawconicgradient(_:_:_:_:))

[justvanrossum]

If I disable the "CGContextDrawConicGradient" codepath, there is still some transparency:

But I'm not sure it's worth fixing, given that even 10.14 (when CGContextDrawConicGradient was introduced) is pretty rare these day, let alone 10.13, which would be the only macOS where this problem exists.

[justvanrossum]

FontGoggles requires macOS >= 10.13.

It's good to keep the fallback around, I'll try to think how we could make it be part of the tests. Shouldn't be a blocker.

[justvanrossum]

Great work again, thanks!