Rework gradient support to match Peniko

[waywardmonkeys]

Peniko has an additional gradient kind that isn't supported here.

We should grow to match Peniko: https://github.com/linebender/peniko/blob/3d040a9d4f309c2dbefbd96b64bfb0cc629fec33/src/gradient.rs#L61-L93

[simbleau]

This is great actually, because Lottie has sweep gradients we never supported :)