allow null flutterTextureId in FlutterFilamentTexture

flutter_filament_federated/flutter_filament_platform_interface/lib/flutter_filament_texture.dart

@@ -1,9 +1,10 @@
 class FlutterFilamentTexture {
   final int width;
   final int height;
-  final int flutterTextureId;
+  final int? flutterTextureId;
   final int? hardwareTextureId;
   final int? surfaceAddress;
+  bool get usesBackingWindow => flutterTextureId == null;
 
   FlutterFilamentTexture(this.flutterTextureId, this.hardwareTextureId,
       this.width, this.height, this.surfaceAddress) {