title: Moving 3D object with mouse and keeping it exactly under the mouse
body: <p>I am using GLScene with Lazarus to render 3D planes (not airplanes) that will act as
  2D bitmaps in the 3D world.</p>
  
  <p>I want to implement mouse events that will let me move objects, which can easily be done.</p>
  
  <p>However, I'm having a hard time figuring out how to keep the object exactly under the mouse
  at the exact same position that I clicked it on.</p>
  
  <p>Scenario:</p>
  
  <p>Lets say I have a plane that is 512x512 pixels in size in 3D-to-2D pixel,
  meaning that even though the object itself is a 3D object, it's position and size
  is represented at exact 2D coordinates to the screen.</p>
  
  <p>If I click on the object at the exact 2D pixel position of 64x64,
  how can I ensure that when I move the mouse, the object is not only moved
  but that it's 64x64 pixel position also stays exactly under the mouse?</p>
  
  <p>Also, how can this be done regardless how far away it is from the camera, e.g it's Z position?</p>