rtmp: support additional Enhanced-RTMP features (#3685) (#4088)

new features:
* support publishing Opus and AC-3 tracks
* support publishing more than 2 tracks. This is compatible with OBS multitrack video and OBS VOD audio track

internal/protocols/rtmp/message/msg_audio_ex_coded_frames.go

@@ -0,0 +1,61 @@
+package message
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/bluenviron/mediamtx/internal/protocols/rtmp/rawmessage"
+)
+
+// AudioExCodedFrames is a CodedFrames extended message.
+type AudioExCodedFrames struct {
+	ChunkStreamID   byte
+	DTS             time.Duration
+	MessageStreamID uint32
+	FourCC          FourCC
+	Payload         []byte
+}
+
+func (m *AudioExCodedFrames) unmarshal(raw *rawmessage.Message) error {
+	if len(raw.Body) < 5 {
+		return fmt.Errorf("not enough bytes")
+	}
+
+	m.ChunkStreamID = raw.ChunkStreamID
+	m.DTS = raw.Timestamp
+	m.MessageStreamID = raw.MessageStreamID
+
+	m.FourCC = FourCC(raw.Body[1])<<24 | FourCC(raw.Body[2])<<16 | FourCC(raw.Body[3])<<8 | FourCC(raw.Body[4])
+	switch m.FourCC {
+	case FourCCOpus, FourCCAC3, FourCCMP4A, FourCCMP3:
+	default:
+		return fmt.Errorf("unsupported fourCC: %v", m.FourCC)
+	}
+
+	m.Payload = raw.Body[5:]
+
+	return nil
+}
+
+func (m AudioExCodedFrames) marshalBodySize() int {
+	return 5 + len(m.Payload)
+}
+
+func (m AudioExCodedFrames) marshal() (*rawmessage.Message, error) {
+	body := make([]byte, m.marshalBodySize())
+
+	body[0] = (9 << 4) | byte(AudioExTypeCodedFrames)
+	body[1] = uint8(m.FourCC >> 24)
+	body[2] = uint8(m.FourCC >> 16)
+	body[3] = uint8(m.FourCC >> 8)
+	body[4] = uint8(m.FourCC)
+	copy(body[5:], m.Payload)
+
+	return &rawmessage.Message{
+		ChunkStreamID:   m.ChunkStreamID,
+		Timestamp:       m.DTS,
+		Type:            uint8(TypeAudio),
+		MessageStreamID: m.MessageStreamID,
+		Body:            body,
+	}, nil
+}