internal: store bounding box with SceneAsset rather than recalculating from renderables

thermion_dart/native/include/c_api/TSceneAsset.h

@@ -21,6 +21,7 @@ extern "C"
     EMSCRIPTEN_KEEPALIVE TSceneAsset *SceneAsset_getInstance(TSceneAsset *tSceneAsset, int index);
     EMSCRIPTEN_KEEPALIVE size_t SceneAsset_getInstanceCount(TSceneAsset *tSceneAsset);
     EMSCRIPTEN_KEEPALIVE TSceneAsset * SceneAsset_createInstance(TSceneAsset *asset, TMaterialInstance **materialInstances, int materialInstanceCount);
+    EMSCRIPTEN_KEEPALIVE Aabb3 SceneAsset_getBoundingBox(TSceneAsset *asset);
     
 #ifdef __cplusplus
 }