Make effects work for other rendermodes than Offscreen

Currently post processing effects are only able to be enabled when the View3D is in Offscreen mode.  Usage of post processing effects do indeed mean that the 3D scene has to be rendered to an FBO, but the render mode of View3D is about where the end target of rendering is.  When using a post processing effect (or chain of PP effects) the last effect used targets the render target, which should be whatever View3D wants it to be.