Fix SortMediaCapability offerer-priority track inversion (#1558)

The ternary in SetRemoteDescription selecting the priority-order key is
flipped relative to the comment above it ("The offerer gets to set the
codec priority"). The current code picks the answerer's track on both
branches:

  - Receiving offer:  uses LocalTrack  (receiver is local; offerer is REMOTE)
  - Receiving answer: uses RemoteTrack (answerer is remote; offerer is LOCAL)

Flipping the branches aligns behavior with the comment.

Symptom: two peers with identical multi-codec audio Capabilities
[Opus, PCMU, PCMA, G722] exchange offer/answer. OnAudioFormatsNegotiated
fires with PCMU 8kHz mono on the offerer and OPUS 48kHz stereo on the
answerer from the same negotiation. With this fix both land on Opus.

Author: LostBeard

src/SIPSorcery/net/RTP/RTPSession.cs

@@ -1218,8 +1218,10 @@ public virtual SetDescriptionResultEnum SetRemoteDescription(SdpType sdpType, SD
                         capabilities = SDPAudioVideoMediaFormat.GetCompatibleFormats(currentMediaStream.RemoteTrack?.Capabilities, currentMediaStream.LocalTrack?.Capabilities);
 
                         // The offerer gets to set the codec priority.
+                        // When receiving an offer: the REMOTE party is the offerer, so their Capabilities are the priority key.
+                        // When receiving an answer: LOCAL is the offerer (we sent the offer), so our Capabilities are the priority key.
                         SDPAudioVideoMediaFormat.SortMediaCapability(capabilities,
-                            sdpType == SdpType.offer ? currentMediaStream.LocalTrack?.Capabilities : currentMediaStream.RemoteTrack?.Capabilities);
+                            sdpType == SdpType.offer ? currentMediaStream.RemoteTrack?.Capabilities : currentMediaStream.LocalTrack?.Capabilities);
 
                         currentMediaStream.LocalTrack.Capabilities = capabilities;
                         currentMediaStream.RemoteTrack.Capabilities = capabilities;