pass through instance owner to FFIAsset so instances can be created on instances

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

@@ -31,12 +31,10 @@ class FFIAsset extends ThermionAsset {
   ///
   ///
   ///
-  FFIAsset? _highlight;
+  bool get isInstance => instanceOwner != null;
+  final FFIAsset? instanceOwner;
+
 
-  ///
-  ///
-  ///
-  final bool isInstance;
 
   ///
   ///
@@ -51,7 +49,7 @@ class FFIAsset extends ThermionAsset {
   ///
   ///
   FFIAsset(this.asset, this.app, this.animationManager,
-      {this.isInstance = false, this.keepData = false}) {
+      {this.instanceOwner = null, this.keepData = false}) {
     entity = SceneAsset_getEntity(asset);
   }
 
@@ -134,9 +132,8 @@ class FFIAsset extends ThermionAsset {
   @override
   Future<FFIAsset> createInstance(
       {covariant List<MaterialInstance>? materialInstances = null}) async {
-    if(!isInstance) {
-      throw Exception(
-          "createInstance cannot be called on an instance. Make sure you are all calling the method on the original asset");
+    if(isInstance) {
+      return instanceOwner!.createInstance(materialInstances: materialInstances);
     }
     if (!keepData) {
       throw Exception(
@@ -171,7 +168,7 @@ class FFIAsset extends ThermionAsset {
     if (created == nullptr) {
       throw Exception("Failed to create instance");
     }
-    return FFIAsset(created, app, animationManager, isInstance: true, keepData: keepData);
+    return FFIAsset(created, app, animationManager, instanceOwner: this, keepData: keepData);
   }
 
   ///

thermion_dart/test/instancing_tests.dart

@@ -77,7 +77,9 @@ void main() async {
       var asset = await viewer.loadGltf(
           "file://${testHelper.testDir}/assets/cube.glb",
           addToScene: false,
-          numInstances: 2, keepData: true);
+          numInstances: 2,
+          keepData: true
+        );
       var defaultInstance = await asset.getInstance(0);
       await viewer.addToScene(defaultInstance);
       await testHelper.capture(viewer.view, "gltf_without_instance");