Make parsing of H.264 NALUs optional

By default, the H264Packet.Unmarshal performs reassembly
of FU-A NALUs.  Not only is this ineficient, it also assumes
that no packet reordering or packet loss ever happens.

Make this optional.

codecs/h264_packet.go

@@ -182,12 +182,23 @@ func (p *H264Payloader) Payload(mtu uint16, payload []byte) [][]byte {
 
 // H264Packet represents the H264 header that is stored in the payload of an RTP Packet
 type H264Packet struct {
-	IsAVC     bool
-	fuaBuffer []byte
+	IsAVC         bool
+	fuaBuffer     []byte
+	dontParseBody bool
 
 	videoDepacketizer
 }
 
+// SetDontParseBody specifies whether to skip parsing the body.
+// If set to true, Unmarshal returns the raw packet body.  If
+// set to false (the default), Unmarshal performs reassembly of
+// FU-A NALUs, which is inefficient and only works if the packets
+// arrive in order.  It is recommended that applications set this
+// to false and perform their own fragment reassembly.
+func (p *H264Packet) SetDontParseBody(dont bool) {
+	p.dontParseBody = true
+}
+
 func (p *H264Packet) doPackaging(buf, nalu []byte) []byte {
 	if p.IsAVC {
 		buf = binary.BigEndian.AppendUint32(buf, uint32(len(nalu)))
@@ -208,6 +219,14 @@ func (p *H264Packet) IsDetectedFinalPacketInSequence(rtpPacketMarketBit bool) bo
 
 // Unmarshal parses the passed byte slice and stores the result in the H264Packet this method is called upon
 func (p *H264Packet) Unmarshal(payload []byte) ([]byte, error) {
+	if p.dontParseBody {
+		return payload, nil
+	}
+
+	return p.parseBody(payload)
+}
+
+func (p *H264Packet) parseBody(payload []byte) ([]byte, error) {
 	if len(payload) == 0 {
 		return nil, fmt.Errorf("%w: %d <=0", errShortPacket, len(payload))
 	}