Conic gradients

[Keavon]

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:

• <foreignObject><div style="background: conic-gradient(red, green, blue)"></div></foreignObject>
• 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