Fix ServerCompositionSimplePen memory leak (#19958)

* Fix for Issue#16451

* Remove the pen from the brush observers without queuing the pen for invalidation.

src/Avalonia.Base/Rendering/Composition/Drawing/ServerCompositionSimplePen.cs

@@ -1,12 +1,19 @@
-using System;
 using Avalonia.Media;
-using Avalonia.Media.Immutable;
-using Avalonia.Rendering.Composition.Server;
-using Avalonia.Rendering.Composition.Transport;
 
 namespace Avalonia.Rendering.Composition.Server;
 
 internal partial class ServerCompositionSimplePen : IPen
 {
     IDashStyle? IPen.DashStyle => DashStyle;
-}
+
+    /// <inheritdoc/>
+    public override void Dispose()
+    {
+        // Remove the pen from the brush observers.
+        // Without this, the pen was being retained in memory by long lived brush resources (e.g. those defined in
+        // the theme or app resources), hence was causing memory leaks; see Issue #16451
+        RemoveObserversFromProperty(ref _brush);
+        _brush = null;
+        base.Dispose();
+    }
+}