separate getter/create method for boundingBoxAsset

thermion_dart/lib/src/filament/src/asset.dart

@@ -45,7 +45,8 @@ abstract class ThermionAsset {
   ///
   ///
   ///
-  Future<MaterialInstance> getMaterialInstanceAt({ThermionEntity? entity, int index = 0});
+  Future<MaterialInstance> getMaterialInstanceAt(
+      {ThermionEntity? entity, int index = 0});
 
   ///
   ///
@@ -64,9 +65,19 @@ abstract class ThermionAsset {
   Future removeStencilHighlight();
 
   ///
-  /// Creates/returns a wireframe asset representing the bounding box for this asset.
+  /// The bounding box for this asset, as an actual renderable asset.
+  /// Null by default; call [createBoundingBoxAsset] first to create.
   ///
-  Future<ThermionAsset> getBoundingBoxAsset();
+  ThermionAsset? get boundingBoxAsset;
+
+  ///
+  /// Creates the renderable bounding box for this asset.
+  /// This is safe to call multiple times; if [boundingBoxAsset] is non-null, 
+  /// this will simply return the existing bounding box asset.
+  /// 
+  /// You will still need to call [Scene.add] to add this to the scene.
+  ///
+  Future<ThermionAsset> createBoundingBoxAsset();
 
   ///
   ///

thermion_dart/lib/src/viewer/src/ffi/src/background_image.dart

@@ -343,8 +343,10 @@ class BackgroundImage extends ThermionAsset {
     throw UnimplementedError();
   }
   
+  ThermionAsset? get boundingBoxAsset => throw UnimplementedError();
+  
   @override
-  Future<ThermionAsset> getBoundingBoxAsset() {
+  Future<ThermionAsset> createBoundingBoxAsset() {
     // TODO: implement getBoundingBoxAsset
     throw UnimplementedError();
   }

thermion_dart/lib/src/viewer/src/ffi/src/ffi_asset.dart

@@ -246,7 +246,7 @@ class FFIAsset extends ThermionAsset {
     throw UnimplementedError();
   }
 
-  FFIAsset? boundingBoxAsset;
+  ThermionAsset? boundingBoxAsset;
 
   Future<v64.Aabb3> getBoundingBox() async {
     final entities = <ThermionEntity>[];
@@ -272,8 +272,7 @@ class FFIAsset extends ThermionAsset {
   ///
   ///
   ///
-  @override
-  Future<ThermionAsset> getBoundingBoxAsset() async {
+  Future<ThermionAsset> createBoundingBoxAsset() async {
     if (boundingBoxAsset == null) {
       final boundingBox = await SceneAsset_getBoundingBox(asset);
 
@@ -365,6 +364,10 @@ class FFIAsset extends ThermionAsset {
     return boundingBoxAsset!;
   }
 
+  ///
+  ///
+  ///
+  @override
   Future<MaterialInstance> getMaterialInstanceAt(
       {ThermionEntity? entity, int index = 0}) async {
     entity ??= this.entity;