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_video_ex_sequence_end.go

@@ -0,0 +1,53 @@
+package message
+
+import (
+	"fmt"
+
+	"github.com/bluenviron/mediamtx/internal/protocols/rtmp/rawmessage"
+)
+
+// VideoExSequenceEnd is a sequence end extended message.
+type VideoExSequenceEnd struct {
+	ChunkStreamID   byte
+	MessageStreamID uint32
+	FourCC          FourCC
+}
+
+func (m *VideoExSequenceEnd) unmarshal(raw *rawmessage.Message) error {
+	if len(raw.Body) != 5 {
+		return fmt.Errorf("not enough bytes")
+	}
+
+	m.ChunkStreamID = raw.ChunkStreamID
+	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 FourCCAV1, FourCCVP9, FourCCHEVC, FourCCAVC:
+	default:
+		return fmt.Errorf("unsupported fourCC: %v", m.FourCC)
+	}
+
+	return nil
+}
+
+func (m VideoExSequenceEnd) marshalBodySize() int {
+	return 5
+}
+
+func (m VideoExSequenceEnd) marshal() (*rawmessage.Message, error) {
+	body := make([]byte, m.marshalBodySize())
+
+	body[0] = 0b10000000 | byte(VideoExTypeSequenceEnd)
+	body[1] = uint8(m.FourCC >> 24)
+	body[2] = uint8(m.FourCC >> 16)
+	body[3] = uint8(m.FourCC >> 8)
+	body[4] = uint8(m.FourCC)
+
+	return &rawmessage.Message{
+		ChunkStreamID:   m.ChunkStreamID,
+		Type:            uint8(TypeVideo),
+		MessageStreamID: m.MessageStreamID,
+		Body:            body,
+	}, nil
+}