Typically this is done to implement a heads-up display (HUD).

  In the draw function callback, the basic algorithm is:

  	save state with pfPushState()
  	disable textures, fog, & lighting with pfBasicState()
  	save & clear projection matrix
  	ortho2()
  	save & clear modelling matrix
  	draw()
  	restore modelling matrix
  	restore projection matrix
  	restore state with pfPopState()

  Or, you can draw your static info in the overlay planes and only
  redraw it when the window receives a REDRAW event (moved, resized,
  etc.).  Changing between drawing to the overlays and drawing to
  regular bitplanes takes a big hit.

  For things that need to be updated real-time, draw() would consist
  of:

          zfunction(ZF_ALWAYS);
          zwritemask(0x0);
  	draw HUD stuff
          zfunction(ZF_LEQUAL);
          zwritemask(0xffffffff);