GitLab

Summary:
- Updated the C++/CUDA mesh rasterization kernels to handle the clipped faces. In particular this required careful handling of the distance calculation for faces which are cut into a quadrilateral by the image plane and then split into two sub triangles i.e. both sub triangles can't be part of the top K faces.
- Updated `rasterize_meshes.py` to use the utils functions to clip the meshes and convert the fragments back to in terms of the unclipped mesh
- Added end to end tests

Reviewed By: jcjohnson

Differential Revision: D26169685

fbshipit-source-id: d64cd0d656109b965f44a35c301b7c81f451cfa0

Summary:
Instead of culling faces behind the camera, partially clip them if they intersect with the image plane.

This diff implements the utils functions for clipping.

There are 4 cases for the mesh faces which are all handled:

```
Case 1: the triangle is completely in front of the clipping plane (it is left
        unchanged)
Case 2: the triangle is completely behind the clipping plane (it is culled)
Case 3: the triangle has exactly two vertices behind the clipping plane (it is
        clipped into a smaller triangle)
Case 4: the triangle has exactly one vertex behind the clipping plane (it is clipped
        into a smaller quadrilateral and divided into two triangular faces)
```

Reviewed By: jcjohnson

Differential Revision: D23108673

fbshipit-source-id: 550a8b6a982d06065dff10aba10d47e8b144ae52