Add backbuffer example with simple shader only scene

Hi, thanks for the great library. I am looking to use wagner to make a live glsl node based editor. I am currently stuck on what would be the best way to chain a bunch of shaders without rendering a scene first. Simply a two triangle plane given a sequence of shaders. I would like the first pass to receive the texture from the last pass (backbuffer). An example of this would be great. Thanks!
