avalonia never resizes so use intptr for update rect.

src/Windows/Avalonia.Win32/Composition/CompositionHost.cs

@@ -39,6 +39,7 @@ namespace Avalonia.Win32
             private readonly IEglWindowGlPlatformSurfaceInfo _info;
             private PixelSize _initialSize;
             private readonly ICompositionDrawingSurfaceInterop _surfaceInterop;
+            private static Guid s_Iid = Guid.Parse("6f15aaf2-d208-4e89-9ab4-489535d34f9c");
 
             public CompositionRenderTarget(EglDisplay display, EglContext context, ICompositionDrawingSurfaceInterop interopSurface, IEglWindowGlPlatformSurfaceInfo info) : base(display, context)
             {
@@ -56,13 +57,11 @@ namespace Avalonia.Win32
             POINT lastSize;
             public override IGlPlatformSurfaceRenderingSession BeginDraw()
             {
-                var iid = Guid.Parse("6f15aaf2-d208-4e89-9ab4-489535d34f9c");
-                var updateRect = new RECT { right = _info.Size.Width, bottom = _info.Size.Height };
                 var offset = new POINT();
            
                 _surfaceInterop.BeginDraw(
-                    ref updateRect,
-                    ref iid,
+                    IntPtr.Zero,
+                    ref s_Iid,
                     out IntPtr texture, ref offset);
 
                 var surface = (_display as AngleWin32EglDisplay).WrapDirect3D11Texture(texture, offset.X, offset.Y, _info.Size.Width, _info.Size.Height);

src/Windows/Avalonia.Win32/Composition/ICompositionDrawingSurfaceInterop.cs

@@ -25,7 +25,7 @@ namespace Windows.UI.Composition.Interop
     [Guid("FD04E6E3-FE0C-4C3C-AB19-A07601A576EE")]
     public interface ICompositionDrawingSurfaceInterop
     {
-        void BeginDraw(ref RECT updateRect, ref Guid iid, out IntPtr updateObject, ref POINT point);
+        void BeginDraw(IntPtr updateRect, ref Guid iid, out IntPtr updateObject, ref POINT point);
 
         void EndDraw();
 
@@ -51,9 +51,9 @@ namespace ABI.Windows.UI.Composition.Interop
 
     {
         [Guid("FD04E6E3-FE0C-4C3C-AB19-A07601A576EE")]
-        public struct Vftbl
+        public unsafe struct Vftbl
         {
-            public delegate int _BeginDraw(IntPtr ThisPtr, ref RECT updateRect, ref Guid iid, out IntPtr updateObject, ref POINT updateOffset);
+            public delegate int _BeginDraw(IntPtr ThisPtr, IntPtr updateRect, ref Guid iid, out IntPtr updateObject, ref POINT updateOffset);
             public delegate int _EndDraw(IntPtr ThisPtr);
             public delegate int _Resize(IntPtr ThisPtr, POINT sizePixels);
             public delegate int _ResumeDraw(IntPtr ThisPtr);
@@ -87,7 +87,7 @@ namespace ABI.Windows.UI.Composition.Interop
                 Marshal.StructureToPtr(AbiToProjectionVftable, AbiToProjectionVftablePtr, false);
             }
 
-            private static int Do_Abi_BeginDraw(IntPtr ThisPtr, ref RECT updateRect, ref Guid iid, out IntPtr updateObject, ref POINT updateOffset)
+            private static int Do_Abi_BeginDraw(IntPtr ThisPtr, IntPtr updateRect, ref Guid iid, out IntPtr updateObject, ref POINT updateOffset)
             {
                 updateObject = IntPtr.Zero;
                 return 0;
@@ -118,9 +118,9 @@ namespace ABI.Windows.UI.Composition.Interop
             _obj = obj;
         }
 
-        public void BeginDraw(ref RECT updateRect, ref Guid iid, out IntPtr updateObject, ref POINT point)
+        public void BeginDraw(IntPtr updateRect, ref Guid iid, out IntPtr updateObject, ref POINT point)
         {
-            Marshal.ThrowExceptionForHR(_obj.Vftbl.BeginDraw(ThisPtr, ref updateRect, ref iid, out updateObject, ref point));
+            Marshal.ThrowExceptionForHR(_obj.Vftbl.BeginDraw(ThisPtr, updateRect, ref iid, out updateObject, ref point));
         }
 
         public void EndDraw()