use surface normals in likelihood noise model

[mattjj]

Compute surface normals in CUDA rendering and use them to better model the noise/corruption/missing data in the kinect images. In particular, surfaces with normals facing up (towards the camera) should have relatively low noise, while surface normals tilted away from the vertical should be noisier, either with a larger added Laplacian noise or with a mixture model for 'disappearance'.