Just added an "Edit2" to my last post. Found that out already. Interesting idea though.
So SSR and PBR are post processing effects and not material effects, I see.
So if one want to render stuff not directly visible to the camera (like a mirror) one has to use something like the water in Urho does? I guess one could also do something like the portals in Portal with that.
Maybe I should make a new thread but how to read the depth buffer? Is there even one? That would be also good to make a depth blur effect but as I tried that there seemed to be no depth information.