add extension method to convert between RECT and PixelRect.

src/Windows/Avalonia.Win32/Win32TypeExtensions.cs

@@ -0,0 +1,13 @@
+using static Avalonia.Win32.Interop.UnmanagedMethods;
+
+namespace Avalonia.Win32
+{
+    public static class Win32TypeExtensions
+    {
+        internal static PixelRect ToPixelRect(this RECT rect)
+        {
+            return new PixelRect(rect.left, rect.top, rect.right - rect.left,
+                    rect.bottom - rect.top);
+        }
+    }
+}