message: copy layer when mirroring model
diff: @@ -215,6 +215,10 @@ namespace MakerJs.model {
  newModel.type = modelToMirror.type;
  }
  + if ('layer' in modelToMirror) {
  + newModel.layer = modelToMirror.layer;
  + }
  +
  if (modelToMirror.units) {
  newModel.units = modelToMirror.units;
  }
  