Add missing a=ssrc lines for RTX/FEC

Pion extracts them from ssrc-group line so this didn't failed
Pion<->Pion. Chrome/FireFox uses the dedicated a=ssrc lines and our
Offer/Answer would break negotation

Fixes #2922

sdp.go

@@ -395,14 +395,20 @@ func addSenderSDP(
 			if encoding.RTX.SSRC != 0 {
 				media = media.WithValueAttribute("ssrc-group", fmt.Sprintf("FID %d %d", encoding.SSRC, encoding.RTX.SSRC))
 			}
-
 			if encoding.FEC.SSRC != 0 {
 				media = media.WithValueAttribute("ssrc-group", fmt.Sprintf("FEC-FR %d %d", encoding.SSRC, encoding.FEC.SSRC))
 			}
 
 			media = media.WithMediaSource(uint32(encoding.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
 
 			if !isPlanB {
+				if encoding.RTX.SSRC != 0 {
+					media = media.WithMediaSource(uint32(encoding.RTX.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
+				}
+				if encoding.FEC.SSRC != 0 {
+					media = media.WithMediaSource(uint32(encoding.FEC.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
+				}
+
 				media = media.WithPropertyAttribute("msid:" + track.StreamID() + " " + track.ID())
 			}
 		}