Fixed RTB Dispose

src/Avalonia.Base/Media/Imaging/RenderTargetBitmap.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Media.Imaging
     /// <summary>
     /// A bitmap that holds the rendering of a <see cref="Visual"/>.
     /// </summary>
-    public class RenderTargetBitmap : Bitmap, IDisposable
+    public class RenderTargetBitmap : Bitmap
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="RenderTargetBitmap"/> class.
@@ -68,5 +68,11 @@ namespace Avalonia.Media.Imaging
             platform.Clear(Colors.Transparent);
             return new PlatformDrawingContext(platform);
         }
+
+        public override void Dispose()
+        {
+            PlatformImpl.Dispose();
+            base.Dispose();
+        }
     }
 }