Increase FEC resilliancy with low traffic

Duplicate first packet to increase resilliancy in cases when the traffic
is low, usually a single packet of some inter-frame compression like
H.264/HEVC. But it will similarly do the job when more packets per frame
are used.

First packet is duplicated instead of the last one because the last packet
can have less symbols than the first if there is more than 1
packet, eg. `DDDD|DF` (D - primary data; F - FEC, | - packet bounadry).

refers to CESNETGH-361

src/transmit.cpp

@@ -687,13 +687,13 @@ tx_send_base(struct tx *tx, struct video_frame *frame, struct rtp *rtp_session,
         }
 
         vector<int> packet_sizes = get_packet_sizes(frame, substream, tx->mtu - hdrs_len);
-        const long mult_pkt_cnt = (long) packet_sizes.size() * tx->mult_count;
+        long mult_pkt_cnt = (long) packet_sizes.size() * tx->mult_count;
         const long packet_rate =
             get_packet_rate(tx, frame, (int) substream, mult_pkt_cnt);
 
         // initialize header array with values (except offset which is different among
         // different packts)
-        void *rtp_headers = malloc(mult_pkt_cnt * rtp_hdr_len);
+        void *rtp_headers = malloc((mult_pkt_cnt + 1) * rtp_hdr_len);
         uint32_t *rtp_hdr_packet = (uint32_t *) rtp_headers;
         for (int m = 0; m < tx->mult_count; ++m) {
                 unsigned pos = 0;
@@ -704,6 +704,11 @@ tx_send_base(struct tx *tx, struct video_frame *frame, struct rtp *rtp_session,
                         pos += packet_sizes.at(i);
                 }
         }
+        if (frame->fec_params.type != FEC_NONE) { // dup 1st pkt with RS/LDGM
+                mult_pkt_cnt += 1;
+                memcpy(rtp_hdr_packet, rtp_hdr, rtp_hdr_len);
+                rtp_hdr_packet[1] = htonl(0);
+        }
 
         if (!tx->encryption) {
                 rtp_async_start(rtp_session, mult_pkt_cnt);