Added support for off screen render targets in the render paths, and using these render targets as textures. Also added fullscreen quad command which means post processing is now possible.
Added an example ForwardGrayscale render path. The code is still not complete yet: For now all generated frame buffers has their own depth texture but they should be shared when possible (if same size)
Also all the code of processing the render path is in the scene class where it does not belong.