fix: Reset external tracks when unloading in cast proxy (#8573)

Author: avelad

lib/cast/cast_proxy.js

@@ -360,6 +360,14 @@ shaka.cast.CastProxy = class extends shaka.util.FakeEventTarget {
     this.playerEventTarget_ = new shaka.util.FakeEventTarget();
     this.playerEventTarget_.dispatchTarget =
       /** @type {EventTarget} */(this.playerProxy_);
+
+    this.eventManager_.listen(this.localPlayer_,
+        shaka.util.FakeEvent.EventName.Unloading, () => {
+          if (this.sender_ && this.sender_.isCasting()) {
+            return;
+          }
+          this.resetExternalTracks();
+        });
   }