DensityPlot SVG generation

[sn6uv]

The gradient mesh for DensityPlot provides some special challenges. Ideally we want to use an SVG gradient mesh but SVG2.0 isn't out yet. We have to make do with linear and radial gradients only, and compose them appropriately.

There is an existing js solution in mathics.js but it would be nice if the backend could output correct svg.

See also #1.

[sn6uv]

@jaycech3n this is the context of the computational geometry problem I was telling you about