MLT renderer: Some progress

Cornell Box scene as a test

Will get rid of those bright dots later. But finally the light source looks right.

Plans for next:

1) Spectral Rendering

    There is already some support for it built in PBRT.  Just needs to figure it out how to implement it.

2) Daylight Sky Model

    Preetham, Shirley and Smits' physically based sky model.  Both Maxwell Render and Indigo use this model. Very realistic skylight     especially when combined with spectral rendering !!

3) Distributed Rendering

         To improve the render time, it has to be parallelized, and jobs can be distributed to some remote machines, e.g., a render farm.

An update for the Sponza scene(render time ~6 or 7 hours)