Remove RTX codec if no primary

While adding transceivers from SetRemoteDescription,
the filtered codecs could filter out the primart codec
and leave the RTX codec in. Generating an answer with
that fails `SetRemoteDescription` on remote peer due
to an unrecognisable codec. Fix it by filtering out
RTX is primary is not there.

rtptransceiver.go

@@ -8,6 +8,7 @@ package webrtc
 
 import (
 	"fmt"
+	"strings"
 	"sync"
 	"sync/atomic"
 
@@ -60,6 +61,19 @@ func (t *RTPTransceiver) SetCodecPreferences(codecs []RTPCodecParameters) error
 		}
 	}
 
+	// remove RTX codecs if there is no corresponding primary codec
+	for i := len(codecs) - 1; i >= 0; i-- {
+		c := codecs[i]
+		if !strings.EqualFold(c.MimeType, MimeTypeRTX) {
+			continue
+		}
+
+		if isRTX, primaryExists := primaryPayloadTypeForRTXExists(c, codecs); isRTX && !primaryExists {
+			// no primary for RTX, remove the RTX
+			codecs = append(codecs[:i], codecs[i+1:]...)
+		}
+	}
+
 	t.codecs = codecs
 
 	return nil