I should add, this code is in the multi branch of the Vello repo: https://github.com/linebender/vello/tree/multi/

It's not ready to check into main yet for a variety of reasons, one of which is that it only applies to fills, not strokes. The plan for that is to do stroke to fill conversion in a compute shader, but to say that's difficult is an understatement. I do seem to specialize in such things, though.