Add alternate version of FullScreenGodray
Add extra camera for rendering light source full screen quad
Rendering light source using RenderWithShader

- create a quad to represent the light in world space
- RenderWithShader on that quad only using the LightSourceShader
Fix light source replaced shader
Make item mask black
Add FullScreenGodray shader to composite ItemsMask with LightSource
All stages are now somewhat working!