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
