Add frame-by-frame animation.

src/ply-frame-buffer.c

Index: src/ply-frame-buffer.c
diff --git a/src/ply-frame-buffer.c b/src/ply-frame-buffer.c
index 4baa1f2d0217677bf05c7bb9891ad493586d9ca9..5c5e45bce7c34ccac00dea85994575d4aff701d2 100755
--- a/src/ply-frame-buffer.c
+++ b/src/ply-frame-buffer.c
@@ -550,20 +550,20 @@ ply_frame_buffer_fill (ply_frame_buffer_t      *buffer,
     {
       /* 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))