Add linear RGB color space.

[mbostock]

As defined here:

https://www.w3.org/TR/css-color-4/#color-conversion-code https://observablehq.com/@mbostock/lab-and-rgb

function rgb_lrgb1(v) {
  return v <= 0.04045 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4;
}
function lrgb_rgb1(v) {
  return v <= 0.0031308 ? 12.92 * v : 1.055 * (v ** (1 / 2.4)) - 0.055;
}

Related d3/d3-interpolate#66.