Fix AOT incompatible code (#7534)

* Fix AOT incompatible code
Use code patterns which are AOT-friendly. That improves R2R and Native AOT scenarios

src/Avalonia.X11/NativeDialogs/Gtk.cs

@@ -63,7 +63,7 @@ namespace Avalonia.X11.NativeDialogs
         public static IDisposable ConnectSignal<T>(IntPtr obj, string name, T handler)
         {
             var handle = GCHandle.Alloc(handler);
-            var ptr = Marshal.GetFunctionPointerForDelegate((Delegate)(object)handler);
+            var ptr = Marshal.GetFunctionPointerForDelegate<T>(handler);
             using (var utf = new Utf8Buffer(name))
             {
                 var id = g_signal_connect_object(obj, utf, ptr, IntPtr.Zero, 0);

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -862,7 +862,7 @@ namespace Avalonia.Win32.Interop
 
             public void Init()
             {
-                biSize = (uint)Marshal.SizeOf(this);
+                biSize = (uint)sizeof(BITMAPINFOHEADER);
             }
         }
 
@@ -1521,7 +1521,7 @@ namespace Avalonia.Win32.Interop
         internal static Version RtlGetVersion()
         {
             RTL_OSVERSIONINFOEX v = new RTL_OSVERSIONINFOEX();
-            v.dwOSVersionInfoSize = (uint)Marshal.SizeOf(v);
+            v.dwOSVersionInfoSize = (uint)Marshal.SizeOf<RTL_OSVERSIONINFOEX>();
             if (RtlGetVersion(ref v) == 0)
             {
                 return new Version((int)v.dwMajorVersion, (int)v.dwMinorVersion, (int)v.dwBuildNumber);
@@ -1914,7 +1914,7 @@ namespace Avalonia.Win32.Interop
                 get
                 {
                     WINDOWPLACEMENT result = new WINDOWPLACEMENT();
-                    result.Length = Marshal.SizeOf(result);
+                    result.Length = Marshal.SizeOf<WINDOWPLACEMENT>();
                     return result;
                 }
             }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -228,7 +228,7 @@ namespace Avalonia.Win32
                     return new Size(rcWindow.Width, rcWindow.Height) / RenderScaling;
                 }
 
-                DwmGetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_EXTENDED_FRAME_BOUNDS, out var rect, Marshal.SizeOf(typeof(RECT)));
+                DwmGetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_EXTENDED_FRAME_BOUNDS, out var rect, Marshal.SizeOf<RECT>());
                 return new Size(rect.Width, rect.Height) / RenderScaling;
             }
         }
@@ -337,7 +337,7 @@ namespace Avalonia.Win32
         private WindowTransparencyLevel Win8xEnableBlur(WindowTransparencyLevel transparencyLevel)
         {
             var accent = new AccentPolicy();
-            var accentStructSize = Marshal.SizeOf(accent);
+            var accentStructSize = Marshal.SizeOf<AccentPolicy>();
 
             if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur)
             {
@@ -392,7 +392,7 @@ namespace Avalonia.Win32
                 bool canUseAcrylic = Win32Platform.WindowsVersion.Major > 10 || Win32Platform.WindowsVersion.Build >= 19628;
 
                 var accent = new AccentPolicy();
-                var accentStructSize = Marshal.SizeOf(accent);
+                var accentStructSize = Marshal.SizeOf<AccentPolicy>();
 
                 if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur && !canUseAcrylic)
                 {