Fix cdg reference and buffer_hints value:

reference must be 3 because both "fields" are used as reference,
and buffer_hints must include readable since we might memcpy from the old frame.

Originally committed as revision 20923 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/cdgraphics.c

@@ -71,8 +71,9 @@ typedef struct CDGraphicsContext {
 static void cdg_init_frame(AVFrame *frame)
 {
     avcodec_get_frame_defaults(frame);
-    frame->reference = 1;
+    frame->reference = 3;
     frame->buffer_hints = FF_BUFFER_HINTS_VALID    |
+                          FF_BUFFER_HINTS_READABLE |
                           FF_BUFFER_HINTS_PRESERVE |
                           FF_BUFFER_HINTS_REUSABLE;
 }