Make parsing of the OBU list in AV1Packet optional

In Pion, the (*XXXPacket).Unmarshal operation is supposed to be
fast and perform no allocation, as it is sometimes used to check
just a single flag in the packet header.  The AV1Packet
implementation breaks this property by parsing the whole list
of OBUs at Unmarshal time, even though it is likely to be unneeded.

The proper solution would be to remove the parsing of OBUs from
Unmarshal, for consistency with the other packet formats.
Unfortunately, this would break backwards compatibility.  We
therefore introduce a field DontParseOBUs which controls the
parsing.  Unfortunately, this means that the inefficient path
is the default, which smells really badly, but there's not much
we can do about it at this stage.

codecs/av1_packet.go

@@ -131,8 +131,15 @@ type AV1Packet struct {
 	// N: MUST be set to 1 if the packet is the first packet of a coded video sequence, and MUST be set to 0 otherwise.
 	N bool
 
+	// If true, don't populate the OBUElements field.
+	//
+	// Deprecated: this will become the default in a future version.
+	DontParseOBUElements bool
+
 	// Each AV1 RTP Packet is a collection of OBU Elements. Each OBU Element may be a full OBU, or just a fragment of one.
 	// AV1Frame provides the tools to construct a collection of OBUs from a collection of OBU Elements
+	//
+	// Deprecated: use the ParseOBUElements method instead.
 	OBUElements [][]byte
 }
 
@@ -153,12 +160,29 @@ func (p *AV1Packet) Unmarshal(payload []byte) ([]byte, error) {
 		return nil, errIsKeyframeAndFragment
 	}
 
-	currentIndex := uint(1)
-	p.OBUElements = [][]byte{}
+	if !p.DontParseOBUElements {
+		elts, err := p.ParseOBUElements(payload[1:])
+		if err != nil {
+			return nil, err
+		}
+		p.OBUElements = elts
+	}
+
+	return payload[1:], nil
+}
+
+// Parse the OBUs in p
+func (p *AV1Packet) ParseOBUElements(payload []byte) ([][]byte, error) {
+	if p.OBUElements != nil {
+		return p.OBUElements, nil
+	}
+
+	elts := [][]byte{}
 
 	var (
 		obuElementLength, bytesRead uint
 		err                         error
+		currentIndex                uint = 0
 	)
 	for i := 1; ; i++ {
 		if currentIndex == uint(len(payload)) {
@@ -180,9 +204,9 @@ func (p *AV1Packet) Unmarshal(payload []byte) ([]byte, error) {
 		if uint(len(payload)) < currentIndex+obuElementLength {
 			return nil, errShortPacket
 		}
-		p.OBUElements = append(p.OBUElements, payload[currentIndex:currentIndex+obuElementLength])
+		elts = append(elts, payload[currentIndex:currentIndex+obuElementLength])
 		currentIndex += obuElementLength
 	}
 
-	return payload[1:], nil
+	return elts, nil
 }