Refactored Film post-processing to use an extensible filter system

[Morgan McGuire]

I also cleaned up Film and Framebuffer documentation. These changes don't allow you to create new filters using just data at run time, however they do make it much easier to add new post-processing filters to Film.cpp.

I intend to use this feature to add variable-resolution rendering and temporal antialiasing soon. 

-m

Prof. Morgan McGuire

Department of Computer Science

Williams College