Handle non-Simulcast Repair Streams

Same issue with TWCC enabled as 11b887. We need to process the
RTX packets so that we can emit proper reports.

rtpcodingparameters.go

@@ -1,10 +1,17 @@
 package webrtc
 
+// RTPRtxParameters dictionary contains information relating to retransmission (RTX) settings.
+// https://draft.ortc.org/#dom-rtcrtprtxparameters
+type RTPRtxParameters struct {
+	SSRC SSRC `json:"ssrc"`
+}
+
 // RTPCodingParameters provides information relating to both encoding and decoding.
 // This is a subset of the RFC since Pion WebRTC doesn't implement encoding/decoding itself
 // http://draft.ortc.org/#dom-rtcrtpcodingparameters
 type RTPCodingParameters struct {
-	RID         string      `json:"rid"`
-	SSRC        SSRC        `json:"ssrc"`
-	PayloadType PayloadType `json:"payloadType"`
+	RID         string           `json:"rid"`
+	SSRC        SSRC             `json:"ssrc"`
+	PayloadType PayloadType      `json:"payloadType"`
+	RTX         RTPRtxParameters `json:"rtx"`
 }