function isAttachedToScene(object) {
      if (object.parent) {
        return isAttachedToScene(object.parent);
      } else {
        return object === scene;
      }
    }
Return A-Frame attachments of each element's object3D group (e.g., mesh).
Children are flattened by one level, removing the THREE.Group wrapper,
so that non-recursive raycasting remains useful.

Only push children defined as component attachments (e.g., setObject3D),
NOT actual children in the scene graph hierarchy.

@param {Array<Element>} els
@returns {Array<THREE.Object3D>}