title: How to get the dimensions of an image from an URL?
body: <p>I want to get the dimensions of an image from an URL, but I don't want to download le full image.
  For example, for a PNG image :</p>
  
  <ul>
  <li>The width can be read from the 16th byte to the 20th</li>
  <li>The height can be read from the 20th to the 24th</li>
  </ul>
  
  <p>So I want to download / read only the first 24 bytes.
  Is it possible? How? Maybe by using <code>Boost</code>, but I don't have any idea how to do it.</p>
  
  <p>Thanks in advance!</p>
  
  <p>Best regards.</p>