The example was broken in #3635 when the `ActiveCamera` logic was introduced, after which there could only be one active `Camera3d` globally.
Ideally there could be one `Camera3d` per render target, not globally, but that isn't the case yet.
To fix the example, we need to
- don't use `Camera3d` twice, add a new `SecondWindowCamera3d` marker
- add the `CameraTypePlugin::<SecondWindowCamera3d>`
- extract the correct `RenderPhase`s
- add a 3d pass driver node for the secondary camera
Fixes#4378
Co-authored-by: Jakob Hellermann <hellermann@sipgate.de>