Set AVFrame.reference correctly for nuv decoder.

Originally committed as revision 17807 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/nuv.c

@@ -197,7 +197,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
 
     if (keyframe && c->pic.data[0])
         avctx->release_buffer(avctx, &c->pic);
-    c->pic.reference = 1;
+    c->pic.reference = 3;
     c->pic.buffer_hints = FF_BUFFER_HINTS_VALID | FF_BUFFER_HINTS_READABLE |
                           FF_BUFFER_HINTS_PRESERVE | FF_BUFFER_HINTS_REUSABLE;
     result = keyframe ? avctx->get_buffer(avctx, &c->pic) : avctx->reget_buffer(avctx, &c->pic);