Fix RemoteServer not starting rendering (#15374)

src/Avalonia.Controls/Remote/RemoteServer.cs

@@ -7,7 +7,7 @@ using Avalonia.Remote.Protocol;
 namespace Avalonia.Controls.Remote
 {
     [Unstable]
-    public class RemoteServer
+    public class RemoteServer : IDisposable
     {
         private EmbeddableControlRoot _topLevel;
 
@@ -22,6 +22,7 @@ namespace Avalonia.Controls.Remote
         {
             _topLevel = new EmbeddableControlRoot(new EmbeddableRemoteServerTopLevelImpl(transport));
             _topLevel.Prepare();
+            _topLevel.StartRendering();
             //TODO: Somehow react on closed connection?
         }
 
@@ -30,5 +31,11 @@ namespace Avalonia.Controls.Remote
             get => _topLevel.Content;
             set => _topLevel.Content = value;
         }
+
+        public void Dispose()
+        {
+            _topLevel.StopRendering();
+            _topLevel.Dispose();
+        }
     }
 }