How I can invest a viewport horizontally?

How I can invest a viewport horizontally?
I need to simulate a mirror, and for this I have a viewport that uses a camera from another position.
I’m using it as a Head-Up Display, not served with SimpleWaterProcessor.

Use a offscreen buffer, that renders to a texture
-> then just use inverted texture coordiantes for your mirror geometry.

(In the examples is one with a offscreen buffer somewhere)