Outlines flicker if the object layer is RenderLayer::none()

Hello! Thanks for the great work on this crate. I'm observing a flickering issue and would like to check if we can come up with a fix, or if one already exists that I'm not aware of. I've noticed that the outlines tend to flicker if their target object has `RenderLayers::none()` inserted.  Here's a modified `render_layers` [example](https://gist.github.com/xiyuoh/a20f10f1fede82fd51997631d9404015) to illustrate what I'm seeing. The cameras and mesh now start with `RenderLayers::none()` instead of default, and the object `RenderLayer` (with `OBJECT_LAYER_ID`) is removed from the mesh. You'll observe that the outlines are flickering when their target mesh has the `RenderLayer` component with no layers. Flickering does not occur if the `RenderLayer` component is not present at all, but that is not an ideal solution for our use case. If this is intentional, I'd like to also ask if there's a good workaround to mitigate this effect. Thank you!