Freeform gradients

Similar to #2302, we need to find a clever way to generate these within the SVG or SVG + CSS spec. Rasterization is probably the most likely approach here. But here are some ideas:

- Rasterization.
- A point cloud of circles with radial gradients interpolating over alpha, with optional transforms to essentially create 2D Gaussian splats.
- For use cases supporting CSS as part of SVG, we can render quads with linear gradients applied to each edge which generates a quad of a mesh, then the CSS perspective transform would skew the four corners to fit within the mesh adjacent to other quads doing the same thing to build a mesh representing the full freeform gradient.

There are two possible sources of color spread, which can be mixed:
- Point (with a color and radius)


- Path (with a color interpolated along the path, such as being set at each anchor)