don't allow creating instances for GridOverlay asset and only expose a single entity

thermion_dart/lib/src/filament/src/implementation/grid_overlay.dart

@@ -12,16 +12,39 @@ class GridOverlay extends FFIAsset {
   static Future<GridOverlay> create(
       FFIFilamentApp app, Pointer<TAnimationManager> animationManager) async {
     if (_overlay == null) {
-      _gridMaterial ??= FFIMaterial(Material_createGridMaterial(app.engine), app);
-      
+      _gridMaterial ??=
+          FFIMaterial(Material_createGridMaterial(app.engine), app);
+
       final asset = await withPointerCallback<TSceneAsset>((cb) =>
           SceneAsset_createGridRenderThread(
               app.engine, _gridMaterial!.getNativeHandle(), cb));
 
       _overlay = GridOverlay(asset, app, animationManager);
       var materialInstance = await _overlay!.getMaterialInstanceAt();
-      await materialInstance.setParameterFloat3("gridColor", 0.1, 0.1, 0.1);
+      await materialInstance
+          .setTransparencyMode(TransparencyMode.TWO_PASSES_TWO_SIDES);
+      await materialInstance.setCullingMode(CullingMode.NONE);
+
+      await materialInstance.setParameterFloat3("gridColor", 0.3, 0.35, 0.3);
+
+      final ffiAsset =
+          FFIAsset(asset, FilamentApp.instance as FFIFilamentApp, nullptr);
+      await FilamentApp.instance!.setPriority(ffiAsset.entity, 0);
+      for (final child in await ffiAsset.getChildEntities()) {
+        await FilamentApp.instance!.setPriority(child, 7);
+      }
+      // await materialInstance.setParameterFloat("distance", 10.0);
     }
     return _overlay!;
   }
+
+  ///
+  ///
+  ///
+  @override
+  Future<FFIAsset> createInstance(
+      {List<MaterialInstance>? materialInstances = null}) async {
+    throw Exception(
+        "Only a single instance of the grid overlay can be created");
+  }
 }

thermion_dart/native/include/scene/GridOverlay.hpp

@@ -37,6 +37,10 @@ public:
     MaterialInstance** getMaterialInstances() override { return &_materialInstance; }
     size_t getMaterialInstanceCount() override { return 1; }
     
+    utils::Entity getEntity() override {
+        return _gridEntity;
+    }
+
     void addAllEntities(Scene* scene) override;
     void removeAllEntities(Scene* scene) override;
     

thermion_dart/native/src/GridOverlay.cpp

@@ -209,10 +209,7 @@ namespace thermion
 
     SceneAsset *GridOverlay::createInstance(MaterialInstance **materialInstances, size_t materialInstanceCount)
     {
-        auto instance = std::make_unique<GridOverlay>(_engine, _material);
-        auto *raw = instance.get();
-        _instances.push_back(std::move(instance));
-        return reinterpret_cast<SceneAsset *>(raw);
+        return nullptr;
     }
 
     void GridOverlay::addAllEntities(Scene *scene)
@@ -250,7 +247,7 @@ namespace thermion
     }
 
     size_t GridOverlay::getChildEntityCount() { 
-        return 1; 
+        return 0; 
     }
 
     Entity GridOverlay::findEntityByName(const char *name)