Seal unlock classes.

src/Avalonia.Native/AvaloniaNativeDeferredRendererLock.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Native
             return null;
         }
 
-        private class UnlockDisposable : IDisposable
+        private sealed class UnlockDisposable : IDisposable
         {
             private IAvnWindowBase _window;
 

src/Avalonia.Visuals/Rendering/ManagedDeferredRendererLock.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Rendering
             return new UnlockDisposable(_lock);
         }
 
-        private class UnlockDisposable : IDisposable
+        private sealed class UnlockDisposable : IDisposable
         {
             private object _lock;