polygonColor = (polygon, faceColor) => {
  let color = faceColor

  if (polygon.shared && polygon.shared.color) {
    color = polygon.shared.color
  } else if (polygon.color) {
    color = polygon.color
  }
  // opaque is default
  if (color !== undefined && color.length < 4) {
    color.push(1.0)
  }
  return color
}
return the color information of a polygon
@param {Object} polygon a csg.js polygon
@param {Object} faceColor a hex color value to default to
@returns {Array}  `[r, g, b, a]`