build image cache in cpp dispEngine

Disp/RenderingCanvas.cs

@@ -253,7 +253,6 @@ namespace Hi.Wpf.Disp
         #endregion
 
         #region DispEngine_Rendering
-        private byte[] PreImageBgra { get; set; }
         /// <summary>
         /// Handles the buffer swapped event from DispEngine
         /// </summary>
@@ -262,12 +261,7 @@ namespace Hi.Wpf.Disp
             if (data == null)
                 return;
 
-            //since the treatment is different from Razor version, 
-            //the image checking mechanism cannot be set in DispEngine.
             Span<byte> bgra = new Span<byte>(data, w * h * 4);
-            if (PreImageBgra != null && bgra.SequenceEqual(PreImageBgra))
-                return;
-            PreImageBgra = bgra.ToArray();
 
             // Copy pixel data from DispEngine
             int n = w * h * 4;