mkCode = function (modules, size, x, y) {
  return (
    mkCodePixel(modules, size, x, y) +
    mkCodePixel(modules, size, x, y + 1)
  )
}
Returns code for four QR pixels. Suitable as key in palette.
@param {boolean[][]} modules
@param {number} size
@param {number} x
@param {number} y
@return {keyof palette}