convert Tracks into Medias and Formats (#155)

* split tracks from medias

* move tracks into dedicated package

* move media into dedicated package

* edit Medias.Marshal() in order to return SDP

* add medias.Find() and simplify examples

* improve coverage

* fix rebase errors

* replace TrackIDs with MediaIDs

* implement media-specific and track-specific callbacks for reading RTCP and RTP packets

* rename publish into record, read into play

* add v2 tag

* rename tracks into formats

examples/client-publish-format-h264/main.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"log"
+	"net"
+
+	"github.com/aler9/gortsplib/v2"
+	"github.com/aler9/gortsplib/v2/pkg/format"
+	"github.com/aler9/gortsplib/v2/pkg/media"
+	"github.com/pion/rtp"
+)
+
+// This example shows how to
+// 1. generate RTP/H264 packets with GStreamer
+// 2. connect to a RTSP server, announce an H264 media
+// 3. route the packets from GStreamer to the server
+
+func main() {
+	// open a listener to receive RTP/H264 packets
+	pc, err := net.ListenPacket("udp", "localhost:9000")
+	if err != nil {
+		panic(err)
+	}
+	defer pc.Close()
+
+	log.Println("Waiting for a RTP/H264 stream on UDP port 9000 - you can send one with GStreamer:\n" +
+		"gst-launch-1.0 videotestsrc ! video/x-raw,width=1920,height=1080" +
+		" ! x264enc speed-preset=veryfast tune=zerolatency bitrate=600000" +
+		" ! rtph264pay ! udpsink host=127.0.0.1 port=9000")
+
+	// wait for first packet
+	buf := make([]byte, 2048)
+	n, _, err := pc.ReadFrom(buf)
+	if err != nil {
+		panic(err)
+	}
+	log.Println("stream connected")
+
+	// create a media that contains a H264 format
+	medi := &media.Media{
+		Type: media.TypeVideo,
+		Formats: []format.Format{&format.H264{
+			PayloadTyp:        96,
+			PacketizationMode: 1,
+		}},
+	}
+
+	// connect to the server and start recording the media
+	c := gortsplib.Client{}
+	err = c.StartRecording("rtsp://localhost:8554/mystream",
+		media.Medias{medi})
+	if err != nil {
+		panic(err)
+	}
+	defer c.Close()
+
+	var pkt rtp.Packet
+	for {
+		// parse RTP packet
+		err = pkt.Unmarshal(buf[:n])
+		if err != nil {
+			panic(err)
+		}
+
+		// route RTP packet to the server
+		err = c.WritePacketRTP(medi, &pkt)
+		if err != nil {
+			panic(err)
+		}
+
+		// read another RTP packet from source
+		n, _, err = pc.ReadFrom(buf)
+		if err != nil {
+			panic(err)
+		}
+	}
+}