Add upstream FlexFEC recovery support

pkg/sfu/buffer/buffer_base.go

@@ -168,6 +168,7 @@ type BufferBase struct {
 	rtpParameters  webrtc.RTPParameters
 	payloadType    uint8
 	rtxPayloadType uint8
+	fecPayloadType uint8
 
 	snRangeMap *utils.RangeMap[uint64, uint64]
 
@@ -313,6 +314,14 @@ func (b *BufferBase) BindLocked(rtpParameters webrtc.RTPParameters, codec webrtc
 		}
 	}
 
+	// find FlexFEC payload type, the repair stream is codec agnostic (no apt)
+	for _, codec := range rtpParameters.Codecs {
+		if strings.EqualFold(codec.MimeType, webrtc.MimeTypeFlexFEC03) {
+			b.fecPayloadType = uint8(codec.PayloadType)
+			break
+		}
+	}
+
 	for _, ext := range rtpParameters.HeaderExtensions {
 		switch ext.URI {
 		case dd.ExtensionURI: