Add av_copy_packet()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -3668,6 +3668,13 @@ int av_grow_packet(AVPacket *pkt, int grow_by);
  */
 int av_dup_packet(AVPacket *pkt);
 
+/**
+ * Copy packet, including contents
+ *
+ * @return 0 on success, negative AVERROR on fail
+ */
+int av_copy_packet(AVPacket *dst, AVPacket *src);
+
 /**
  * Free a packet.
  *