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/common.go

@@ -21,9 +21,17 @@ func (d *audioDepacketizer) IsPartitionHead(_ []byte) bool {
 	return true
 }
 
+func (d *audioDepacketizer) SetZeroAllocation(bool) {}
+
 // videoDepacketizer is a mixin for video codec depacketizers
-type videoDepacketizer struct{}
+type videoDepacketizer struct {
+	zeroAllocation bool
+}
 
 func (d *videoDepacketizer) IsPartitionTail(marker bool, _ []byte) bool {
 	return marker
 }
+
+func (d *videoDepacketizer) SetZeroAllocation(zeroAllocation bool) {
+	d.zeroAllocation = zeroAllocation
+}

codecs/h264_packet.go

@@ -208,6 +208,14 @@ 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.zeroAllocation {
+		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))
 	}

depacketizer.go

@@ -5,11 +5,21 @@ package rtp
 
 // Depacketizer depacketizes a RTP payload, removing any RTP specific data from the payload
 type Depacketizer interface {
+	// Unmarshal parses the RTP payload and returns media.
+	// Metadata may be stored on the Depacketizer itself
 	Unmarshal(packet []byte) ([]byte, error)
+
+	// SetZeroAllocation enables Zero Allocation mode for the depacketizer
+	// By default the Depacketizers will allocate as they parse. These allocations
+	// are needed for Metadata and other optional values. If you don't need this information
+	// enabling SetZeroAllocation gives you higher performance at a reduced feature set.
+	SetZeroAllocation(bool)
+
 	// Checks if the packet is at the beginning of a partition.  This
 	// should return false if the result could not be determined, in
 	// which case the caller will detect timestamp discontinuities.
 	IsPartitionHead(payload []byte) bool
+
 	// Checks if the packet is at the end of a partition.  This should
 	// return false if the result could not be determined.
 	IsPartitionTail(marker bool, payload []byte) bool