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

pkg/format/generic.go

@@ -0,0 +1,111 @@
+package format
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/pion/rtp"
+)
+
+func findClockRate(payloadType uint8, rtpMap string) (int, error) {
+	// get clock rate from payload type
+	// https://en.wikipedia.org/wiki/RTP_payload_formats
+	switch payloadType {
+	case 0, 1, 2, 3, 4, 5, 7, 8, 9, 12, 13, 15, 18:
+		return 8000, nil
+
+	case 6:
+		return 16000, nil
+
+	case 10, 11:
+		return 44100, nil
+
+	case 14, 25, 26, 28, 31, 32, 33, 34:
+		return 90000, nil
+
+	case 16:
+		return 11025, nil
+
+	case 17:
+		return 22050, nil
+	}
+
+	// get clock rate from rtpmap
+	// https://tools.ietf.org/html/rfc4566
+	// a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding parameters>]
+	if rtpMap == "" {
+		return 0, fmt.Errorf("attribute 'rtpmap' not found")
+	}
+
+	tmp := strings.Split(rtpMap, "/")
+	if len(tmp) != 2 && len(tmp) != 3 {
+		return 0, fmt.Errorf("invalid rtpmap (%v)", rtpMap)
+	}
+
+	v, err := strconv.ParseInt(tmp[1], 10, 64)
+	if err != nil {
+		return 0, err
+	}
+
+	return int(v), nil
+}
+
+// Generic is a generic format.
+type Generic struct {
+	PayloadTyp uint8
+	RTPMap     string
+	FMTP       string
+
+	// clock rate of the format. Filled automatically.
+	ClockRat int
+}
+
+// Init initializes a Generic.
+func (t *Generic) Init() error {
+	t.ClockRat, _ = findClockRate(t.PayloadTyp, t.RTPMap)
+	return nil
+}
+
+// String returns a description of the format.
+func (t *Generic) String() string {
+	return "Generic"
+}
+
+// ClockRate implements Format.
+func (t *Generic) ClockRate() int {
+	return t.ClockRat
+}
+
+// PayloadType implements Format.
+func (t *Generic) PayloadType() uint8 {
+	return t.PayloadTyp
+}
+
+func (t *Generic) unmarshal(payloadType uint8, clock string, codec string, rtpmap string, fmtp string) error {
+	t.PayloadTyp = payloadType
+	t.RTPMap = rtpmap
+	t.FMTP = fmtp
+
+	return t.Init()
+}
+
+// Marshal implements Format.
+func (t *Generic) Marshal() (string, string) {
+	return t.RTPMap, t.FMTP
+}
+
+// Clone implements Format.
+func (t *Generic) Clone() Format {
+	return &Generic{
+		PayloadTyp: t.PayloadTyp,
+		RTPMap:     t.RTPMap,
+		FMTP:       t.FMTP,
+		ClockRat:   t.ClockRat,
+	}
+}
+
+// PTSEqualsDTS implements Format.
+func (t *Generic) PTSEqualsDTS(*rtp.Packet) bool {
+	return true
+}