Consider first packet when reading Simulcast IDs (#3144)

The code currently ignores the first packet when reading Simulcast IDs
from a new SSRC, and probes only subsequent packets. This commit makes
it so that we consider the first packet as well (which we already have
read). Helps if the publisher only sends Simulcast IDs on the first
packet.

rtptransceiver.go

@@ -428,10 +428,10 @@ func handleUnknownRTPPacket(
 	streamIDExtensionID,
 	repairStreamIDExtensionID uint8,
 	mid, rid, rsid *string,
-) (payloadType PayloadType, paddingOnly bool, err error) {
+) (paddingOnly bool, err error) {
 	rp := &rtp.Packet{}
 	if err = rp.Unmarshal(buf); err != nil {
-		return 0, false, err
+		return false, err
 	}
 
 	if rp.Padding && len(rp.Payload) == 0 {
@@ -439,10 +439,9 @@ func handleUnknownRTPPacket(
 	}
 
 	if !rp.Header.Extension {
-		return payloadType, paddingOnly, nil
+		return paddingOnly, nil
 	}
 
-	payloadType = PayloadType(rp.PayloadType)
 	if payload := rp.GetExtension(midExtensionID); payload != nil {
 		*mid = string(payload)
 	}
@@ -455,5 +454,5 @@ func handleUnknownRTPPacket(
 		*rsid = string(payload)
 	}
 
-	return payloadType, paddingOnly, nil
+	return paddingOnly, nil
 }