function vgi(pixel) {
  const r = pixel[0] / 255;
  const g = pixel[1] / 255;
  const b = pixel[2] / 255;
  return (2 * g - r - b) / (2 * g + r + b);
}
Calculate the Vegetation Greenness Index (VGI) from an input pixel.  This
is a rough estimate assuming that pixel values correspond to reflectance.
@param {Array<number>} pixel An array of [R, G, B, A] values.
@return {number} The VGI value for the given pixel.