fixes for window resizing on Windows

lib/rendering_surface.dart

@@ -0,0 +1,37 @@
+import 'dart:ffi';
+
+class RenderingSurface {
+  final int flutterTextureId;
+  final Pointer<Void> surface;
+  final int textureHandle;
+  final int sharedContext;
+
+  factory RenderingSurface.from(dynamic platformMessage) {
+    var flutterTextureId = platformMessage[0];
+
+    // void* on iOS (pointer to pixel buffer), Android (pointer to native window), null on macOS/Windows
+    var surfaceAddress = platformMessage[1] as int? ?? 0;
+
+    // null on iOS/Android, void* on MacOS (pointer to metal texture), GLuid on Windows/Linux
+    var nativeTexture = platformMessage[2] as int? ?? 0;
+
+    if(nativeTexture != 0) {       
+      assert(surfaceAddress == 0);
+    }
+
+    var sharedContext = platformMessage[3] as int? ?? 0;
+
+    print(
+        "Using flutterTextureId $flutterTextureId, surface $surfaceAddress nativeTexture $nativeTexture and sharedContext $sharedContext");
+    return RenderingSurface(
+      sharedContext: sharedContext,
+        flutterTextureId: flutterTextureId,
+        surface: Pointer<Void>.fromAddress(surfaceAddress),
+        textureHandle: nativeTexture);
+  }
+
+  RenderingSurface({required this.sharedContext, 
+      required this.flutterTextureId,
+      required this.surface,
+      required this.textureHandle});
+}