Createdepth

Pull request consistent of two parts.
The first is the creation of a new fake surface integrator "depth" that just output the distance of the surface with the camera centre for every point, eventually scaled by "factor" parameter
The second is a script in the tool directory that compute the correct pbrt parameters given a camera matrix P and x and y resolutions.
This is a useful addition to pbrt that makes it possible to create reliable ground truth from mesh models given a camera matrix.