forgetModel method
Forgets the shapes made from model, and what geometry kept of it
when that is an AssetColliderGeometry3D: after the model was loaded
again, its colliders take its new triangles
(PhysicsSystem3D.reloadModel calls this).
Implementation
void forgetModel(String model) {
_models.removeWhere((key, _) => key.$2 == model);
final source = geometry;
if (source is AssetColliderGeometry3D) source.forget(model);
}