function flip (flip) {
  this.options.flip = is.bool(flip) ? flip : true;
  return this;
}
Mirror the image vertically (up-down) about the x-axis.
This always occurs before rotation, if any.

This operation does not work correctly with multi-page images.

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

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