title: Xorg self changing scan-out/rendering buffer before outputting
body: <p>I have non typical question.
  For example I have PC with 2 projectors connected to it.
  I use XRANDR to cat two displays (2 projectors) to one virtual screen.</p>
  
  <p>So, as I understand I have two scan-out buffers for each projector.
  Each buffer contains RGB pixels, that will be outputted via Video Card.</p>
  
  <p>I want to manipulate pixels before it will be scan out, but after xserver will render hole picture.</p>
  
  <p>My question is How to do it? 
  I think I must add a hook somewhere, but where? What level of software I must change?
  Somewhere in the kernel? Or I can change somewhere in xserver side?</p>
  
  <p>All information will be usefull. Thanks!</p>