feat: Add initial-last-n to jingle if config.channelLastN is set. (#2232)

* feat: Add initial-last-n to jingle if config.channelLastN is set.

* Also add initial-last-n to transport-accept.

modules/sdp/SDP.js

             elem.attrs({ name: mid });
         }
 
+        if (mline.media === 'video' && typeof this.initialLastN === 'number') {
+            elem.c('initial-last-n',
+                { xmlns: 'jitsi:colibri2',
+                    value: this.initialLastN }).up();
+        }
+
         if (mline.media === 'audio' || mline.media === 'video') {
             elem.c('description',
                 { xmlns: 'urn:xmpp:jingle:apps:rtp:1',

modules/xmpp/JingleSessionPC.js

                 this.setOfferAnswerCycle(
                     originalOffer,
                     () => {
-                        const localSDP
-                            = new SDP(this.peerconnection.localDescription.sdp);
+                        const localSDP = new SDP(this.peerconnection.localDescription.sdp);
+
+                        if (typeof this.options.channelLastN === 'number' && this.options.channelLastN >= 0) {
+                            localSDP.initialLastN = this.options.channelLastN;
+                        }
 
                         this.sendTransportAccept(localSDP, success, failure);
 
         if (this.failICE) {
             localSDP.failICE = true;
         }
+        if (typeof this.options.channelLastN === 'number' && this.options.channelLastN >= 0) {
+            localSDP.initialLastN = this.options.channelLastN;
+        }
         localSDP.toJingle(
             accept,
             this.initiatorJid === this.localJid ? 'initiator' : 'responder');