avpacket: use AVBuffer to allow refcounting the packets.

This will allow us to avoid copying the packets in many cases.

This breaks ABI.

libavformat/psxstr.c

@@ -201,6 +201,10 @@ static int str_read_packet(AVFormatContext *s,
                     *ret_pkt = *pkt;
                     pkt->data= NULL;
                     pkt->size= -1;
+                    pkt->buf = NULL;
+#if FF_API_DESTRUCT_PACKET
+                    pkt->destruct = NULL;
+#endif
                     return 0;
                 }