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

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