Conic gradients

This could be implemented using a few strategies, and we probably will want multiple so the user can pick what's best for their use case.

For support in all SVG viewers:
- A whole bunch of solid-colored pie slice triangles
- Rasterized as a base64 image at a set resolution

For support in some SVG viewers like browsers:
- `

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- Possibly fewer pie slice triangles made out of linear gradient quads given CSS perspective transforms to turn them into triangles by pinching a pair of corners, however pure SVG only supports affine transforms