code: function customize(bitmap) {
      const size = bitmap.size;
      const data = bitmap.data;
  
      for (let x = 0; x < size; x++) {
          for (let y = 0; y < x; y++) {
              const offset = coord2offset(x, y, size);
              // If it's white change it's color
              if (data[offset]) {
                  data[offset] = 255 - Math.abs(x - y);
              }
          }
      }
  }
docs: bitmap.size — width (and height) of resulting image in pixels
  bitmap.data — Buffer with image data. It's a linear representation
  of image in format:
  <00> <xx> <xx> ..        <xx>
  <00> <xx> <xx> ..        <xx>
  ..
  <00> <xx> <xx> ..        <xx>
  ^    | size number of bytes |
  \ do not modify. Must be 00
  
  Each <xx> is a pixel of image. It's value 0 — black, 255 — white,
  everything between are shades of gray.