Fix SplashScreen fade-out transparency regression after CsWin32 refactor

Initialize _blendFunction instead of using a local BLENDFUNCTION instance.
During fade-out, the uninitialized field caused AlphaFormat to be 0,
resulting in transparent regions rendering gray.

Fixes #11321

Author: sachPundir

src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/SplashScreen.cs

@@ -173,7 +173,7 @@ private unsafe HWND CreateWindow(HBITMAP hBitmap, int width, int height, bool to
             using CreateDcScope memoryContext = new(screenContext);
             using SelectObjectScope selectObjectScope = new(memoryContext, hBitmap);
 
-            BLENDFUNCTION blendFunction = new()
+            _blendFunction = new()
             {
                 BlendOp = (byte)PInvoke.AC_SRC_OVER,
                 SourceConstantAlpha = 255,
@@ -188,7 +188,7 @@ private unsafe HWND CreateWindow(HBITMAP hBitmap, int width, int height, bool to
                 memoryContext,
                 new(0, 0),
                 default,
-                blendFunction,
+                _blendFunction,
                 UPDATE_LAYERED_WINDOW_FLAGS.ULW_ALPHA))
             {
                 ((HRESULT)Marshal.GetHRForLastWin32Error()).ThrowOnFailure();