code: function unflatten(valuesInOneDimension, size) {
    const {height, width} = size;
    const valuesInTwoDimensions = [];
    for (let y = 0; y < height; y++) {
      const start = y * width;
      const end = start + width;
      valuesInTwoDimensions.push(valuesInOneDimension.slice(start, end));
    }
    return valuesInTwoDimensions;
  }
docs: /*
  Takes in a flattened one dimensional array
  representing two-dimensional pixel values
  and returns an array of arrays.