title: Blend FBO onto default framebuffer
body: <p>To clarify, when I say 'default framebuffer' I mean the one provided by the windowing system and what ends up on your monitor.</p>
  
  <p>To improve my rendering speeds for a CAD app, I've managed to separate out the 3D elements from the Qt-handled 2D ones, and they now each render into their own FBO.  When the time comes to get them onto the screen, I blit the 3D FBO onto the default FB, and then I want to blend my 2D FBO on top of it.</p>
  
  <p>I've gotten to the blitting part fine, but I can't see how to blend my 2D FBO onto it?  Both FBOs are identical in size and format, and they are both the same as the default FB.</p>
  
  <p>I'm sure it's a simple operation, but I can't find anything on the net - presumably I'm missing the right term for what I am trying to do.  Although I'm using Qt, I can use native OpenGL commands without issue.</p>