encapsulate and cache os version.

src/Windows/Avalonia.Win32/Win32Platform.cs

@@ -58,6 +58,11 @@ namespace Avalonia.Win32
             CreateMessageWindow();
         }
 
+        /// <summary>
+        /// Gets the actual WindowsVersion. Same as the info returned from RtlGetVersion.
+        /// </summary>
+        public static Version WindowsVersion { get; } = RtlGetVersion();
+
         public static bool UseDeferredRendering => Options.UseDeferredRendering;
         internal static bool UseOverlayPopups => Options.OverlayPopups;
         public static Win32PlatformOptions Options { get; private set; }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -227,15 +227,14 @@ namespace Avalonia.Win32
 
         private WindowTransparencyLevel EnableBlur(WindowTransparencyLevel transparencyLevel)
         {
-            var version = RtlGetVersion();
             bool canUseTransparency = false;
             bool canUseAcrylic = false;
 
-            if (version.Major >= 10)
+            if (Win32Platform.WindowsVersion.Major >= 10)
             {
                 canUseTransparency = true;
 
-                if (version.Build >= 19628)
+                if (Win32Platform.WindowsVersion.Build >= 19628)
                 {
                     canUseAcrylic = true;
                 }