title: OpenGL: projecting mouse click onto geometry
body: <p>I have this view set:</p>
  
  <pre><code>glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
  glLoadIdentity(); //Reset the drawing perspective
  </code></pre>
  
  <p>and I get a screen position (sx, sy) from a mouse click.</p>
  
  <p>Given a value of z, how can I calculate x and y in 3d-space from sx and sy?</p>