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-read-format-g711/main.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"log"
+
+	"github.com/aler9/gortsplib/v2"
+	"github.com/aler9/gortsplib/v2/pkg/format"
+	"github.com/aler9/gortsplib/v2/pkg/url"
+	"github.com/pion/rtp"
+)
+
+// This example shows how to
+// 1. connect to a RTSP server
+// 2. check if there's a G711 media
+// 3. get G711 frames of that media
+
+func main() {
+	c := gortsplib.Client{}
+
+	// parse URL
+	u, err := url.Parse("rtsp://localhost:8554/mystream")
+	if err != nil {
+		panic(err)
+	}
+
+	// connect to the server
+	err = c.Start(u.Scheme, u.Host)
+	if err != nil {
+		panic(err)
+	}
+	defer c.Close()
+
+	// find published medias
+	medias, baseURL, _, err := c.Describe(u)
+	if err != nil {
+		panic(err)
+	}
+
+	// find the G711 media and format
+	var trak *format.G711
+	medi := medias.Find(&trak)
+	if medi == nil {
+		panic("media not found")
+	}
+
+	// setup decoder
+	rtpDec := trak.CreateDecoder()
+
+	// setup the chosen media only
+	_, err = c.Setup(medi, baseURL, 0, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// called when a RTP packet arrives
+	c.OnPacketRTP(medi, trak, func(pkt *rtp.Packet) {
+		// decode a G711 packet from the RTP packet
+		op, _, err := rtpDec.Decode(pkt)
+		if err != nil {
+			return
+		}
+
+		// print
+		log.Printf("received G711 frame of size %d\n", len(op))
+	})
+
+	// start playing
+	_, err = c.Play(nil)
+	if err != nil {
+		panic(err)
+	}
+
+	// wait until a fatal error
+	panic(c.Wait())
+}