title: 4 point image transformation
body: <p>I'm trying to implement something like this: <a href="https://math.stackexchange.com/questions/296794/finding-the-transform-matrix-from-4-projected-points-with-javascript">https://math.stackexchange.com/questions/296794/finding-the-transform-matrix-from-4-projected-points-with-javascript</a></p>
  
  <p>I basically want to transform an image in a perspectively correct way when given 4 points. My knowledge of 3d transformations is fairly limited so I already struggle to get an image positioned correctly while using a <code>PerspectiveCamera</code>.</p>
  
  <p>I dont't need to be able to drag the end points, I just want to easily define that kind of transformation.</p>
  
  <p>The reason I wan't to do this with three.js and not use a css transformation is that I need higher browser support and the capability to save the image afterwards so I thought of using the <code>CanvasRenderer</code>.</p>