Add support for sweep gradients in COLR v1

[PeterCon]

In the current WD, the enhancements for COLR v1 include linear and radial gradient. There is a third type of basic, colour gradient commonly recognized: sweep (or "conical" / "angular") gradients. This gradient type should also be added for COLR v1.

The necessary content revisions have been prepared in this submission: https://github.com/googlefonts/colr-gradients-spec/blob/master/OFF_AMD2_WD.md. Sub-clause 5.7.11.1.2.4 Sweep gradients is added, describing this type of gradient; but there are other changes needed in following parts of the content for 5.7.11.

[PeterConstable]

The draft in the Google Fonts repo has been updated to address this issue. The following is the main pull request for that change:

• Add PaintSweepGradient
• PaintSweepGradient: rename x -> centerX, y -> centerY
• Update sweep gradient per #250

[vlevantovsky]

Proposed changes are introduced by input contribution m56339.