title: Get new position of coordinate after rotation with Matrix
body: <p>I'm wondering how to use a Matrix to get the new position of a coordinate within a rectangle after rotation. What I would like to do is: </p>
  
  <ol>
  <li>Define a rectangle</li>
  <li>Define a coordinate within that rectangle</li>
  <li>Rotate the rectangle</li>
  <li>Get the new position of the coordinate after the rotation</li>
  </ol>
  
  <p>The parts I can't figure out are 2 &amp; 4.
  Any ideas?</p>