title: Get mouse coordinates relative to Qt OpenGL widget
body: <p>I get my mouse coordinates like this:</p>
  
  <pre><code> winX = QCursor::pos().x();
  </code></pre>
  
  <p>These are coordinates for the whole screen. I need to get coordinates that are relative to my OpenGL widget window's viewport, so I can use the gluUnProject function.</p>
  
  <p>How can I do that? </p>