fix ramp legend colorSpace

[mbostock]

This replaces the SVG filter added in #2416 with a simpler color-mix(…) which is available since the ramp legend directly sets context.fillStyle (and hence we don’t have to worry, as we did with the raster mark, about converting a color string into RGBA values). It also allows the caller to set the colorSpace when constructing an opacity legend, so that P3 colors can be used as intended.

I’m not sure why the snapshots aren’t appearing correctly here; the tests seem to be correct locally.

[mbostock]

I think the snapshots don’t render correctly because node-canvas doesn’t support CSS Color Module Level 5 color-mix(…)… But I think this is still a preferable implementation and it should make #2285 easier.

[Fil]

I'm not seeing why it's preferable or simpler than the basic greyscale ramp + svg filter to add the color. (But as long as it works I'm fine with it.)

[mbostock]

It’s simpler because you don’t need the filter — as the diff shows.