Add frame-by-frame animation.

src/ply-frame-buffer.c

 /* ply-frame-buffer.c - framebuffer abstraction
  *
  * Copyright (C) 2006, 2007, 2008 Red Hat, Inc.
  *               2008 Charlie Brej <cbrej@cs.man.ac.uk>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2, or (at your option)
  * any later version.
  *
@@ skipping 532 matching lines @@
   hdiff = area->width - buffer->area.width;
   vdiff = area->height - buffer->area.height;
 
   lines = vdiff > 0 ? buffer->area.height : area->height;
   width = hdiff > 0 ? buffer->area.width : area->width;
 
   if (hdiff >= 0)
     {
       /* image is wider than buffer */
       dst = &buffer->map_address[0];
-      src = (char *) (data + hdiff / 2);
+      src = (char *) (data + hdiff / 2 - x);
     } else {
-      dst = &buffer->map_address[(-hdiff / 2) * buffer->bytes_per_pixel];
+      dst = &buffer->map_address[(-hdiff / 2 + x) * buffer->bytes_per_pixel];
       src = (char *) data;
     }
 
   if (vdiff >= 0)
     {
       /* image is taller than buffer */
-      src += (vdiff / 2) * area->width * sizeof(*data);
+      src += (vdiff / 2 - y) * area->width * sizeof(*data);
     }
   else
     {
-      dst += (-vdiff / 2) * buffer->row_stride * buffer->bytes_per_pixel;
+      dst += (-vdiff / 2 + y) * buffer->row_stride * buffer->bytes_per_pixel;
     }
 
   if (buffer->bytes_per_pixel != sizeof(*data))
     {
       uint16_t *src_16 = 0, *src_16_end;
       int line;
 
       /* Handle the 16 bpp case (specifically RGB 565 case) */
       if (buffer->bytes_per_pixel != 2)
         return 0;
@@ skipping 56 matching lines @@
 
 
 void
 ply_frame_buffer_clear (ply_frame_buffer_t *buffer)
 {
   assert (buffer != NULL);
   assert (ply_frame_buffer_device_is_open (buffer));
   memset (&buffer->map_address[0], 0,
           buffer->area.width * buffer->area.height * buffer->bytes_per_pixel);
 }