DBuff
   =====

   This program is not in the public domain, but it may be freely copied
   and distributed for no charge providing this header is included.
   The code may be modified as required, but any modifications must be
   documented so that the person responsible can be identified. If someone
   else breaks this code, I don't want to be blamed for code that does not
   work! The code may not be sold commercially without prior permission from
   the author, although it may be given away free with commercial products,
   providing it is made clear that this program is free and that the source
   code is provided with the program.

****************************************************************************

   Description:
   ============
   These routines set up, manipulate and tidy up for double buffered
   animation using an Intuition Screen and Window

****************************************************************************

   Usage:
   ======

   5 routines are supplied:

>  RastPort = (struct RastPort *)InitDBuff(GfxBase,Screen,depth,Window)
   --------------------------------------------------------------------
   Given the GfxBase, Screen and Window, this routine returns the
   second RastPort


>  SetView1()
   ----------
>  SetView2()
   ----------
   Sets the required view ready for drawing. SetView1() sets the Intuition
   view, SetView2() sets the alternate view.
   

>  SwapView()
   ----------
   Displays the new view (after drawing)
   

>  FreeDBuff(screen,depth,RastPort)
   --------------------------------
   Frees up the memory, etc. for the second view.



****************************************************************************

   How to use:
   ===========

   1. Declare a Screen pointer, Window pointer and RastPort pointer as
      normal.
   2. Declare a second RastPort pointer.
   3. Open an Intuition screen as normal with a full size NOBORDER window.
   4. Set the first RastPort to the Intuition RastPort:
      rport1 = window->RPort
   5. Set the second RastPort using a call to InitDBuff():
      rport2 = (struct RastPort *)InitDBuff(GfxBase,screen,DEPTH,window);
   6. Make sure we've got the Intuition view by calling SetView1();
   7. Draw into the *second* RastPort, rport2.
   8. Call SwapView() to get this RastPort displayed.
   9. Make sure we've got the second view by calling SetView2();
  10. Draw into the *first* RastPort, rport1.
  11. Call SwapView() to get this RastPort displayed.
  12. If not finished, go to 6.
  13. Free the second ViewPort, etc. by calling:
      FreeDBuff(screen,DEPTH,rport2);


****************************************************************************

   Notes:
   ======

   1. Assumes both graphics and intuition libraries are open
   2. The window should be NOBORDER and have a null title
   3. For maximum speed in swapping views, the SwapView() call could
      be replaced by the following code.
      Put this up the top somewhere:
         #include <graphics/gfxbase.h>
         struct View *DB_view;
         DB_view = GfxBase->ActiView;
      Replace exch SwapView() call with:
         MrgCpr(DB_view);
   4. #define DEMO to see the demonstration.