function flop (flop) {
  this.options.flop = is.bool(flop) ? flop : true;
  return this;
}
Mirror the image horizontally (left-right) about the y-axis.
This always occurs before rotation, if any.

@example
const output = await sharp(input).flop().toBuffer();

@param {Boolean} [flop=true]
@returns {Sharp}