Complete the VP8 unmarshaller

The unmarshaller was ignoring the scalability data.

README.md

@@ -50,6 +50,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [lxb](https://github.com/lxb531)
 * [Robin Raymond](https://github.com/robin-raymond)
 * [debiandebiandebian](https://github.com/debiandebiandebian)
+* [Juliusz Chroboczek](https://github.com/jech)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

codecs/vp8_packet.go

@@ -91,18 +91,23 @@ func (p *VP8Payloader) Payload(mtu int, payload []byte) [][]byte {
 // VP8Packet represents the VP8 header that is stored in the payload of an RTP Packet
 type VP8Packet struct {
 	// Required Header
-	X   uint8 /* extended controlbits present */
-	N   uint8 /* (non-reference frame)  when set to 1 this frame can be discarded */
+	X   uint8 /* extended control bits present */
+	N   uint8 /* when set to 1 this frame can be discarded */
 	S   uint8 /* start of VP8 partition */
 	PID uint8 /* partition index */
 
-	// Optional Header
-	I         uint8  /* 1 if PictureID is present */
-	L         uint8  /* 1 if TL0PICIDX is present */
-	T         uint8  /* 1 if TID is present */
-	K         uint8  /* 1 if KEYIDX is present */
+	// Extended control bits
+	I uint8 /* 1 if PictureID is present */
+	L uint8 /* 1 if TL0PICIDX is present */
+	T uint8 /* 1 if TID is present */
+	K uint8 /* 1 if KEYIDX is present */
+
+	// Optional extension
 	PictureID uint16 /* 8 or 16 bits, picture ID */
 	TL0PICIDX uint8  /* 8 bits temporal level zero index */
+	TID       uint8  /* 2 bits temporal layer index */
+	Y         uint8  /* 1 bit layer sync bit */
+	KEYIDX    uint8  /* 5 bits temporal key frame index */
 
 	Payload []byte
 }
@@ -152,11 +157,27 @@ func (p *VP8Packet) Unmarshal(payload []byte) ([]byte, error) {
 		}
 	}
 
+	if payloadIndex >= payloadLen {
+		return nil, errShortPacket
+	}
+
 	if p.L == 1 {
+		p.TL0PICIDX = payload[payloadIndex]
 		payloadIndex++
 	}
 
+	if payloadIndex >= payloadLen {
+		return nil, errShortPacket
+	}
+
 	if p.T == 1 || p.K == 1 {
+		if p.T == 1 {
+			p.TID = payload[payloadIndex] >> 6
+			p.Y = (payload[payloadIndex] >> 5) & 0x1
+		}
+		if p.K == 1 {
+			p.KEYIDX = payload[payloadIndex] & 0x1F
+		}
 		payloadIndex++
 	}