CUDA/C++ Rasterizer updates to handle clipped faces

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

tests/test_render_implicit.py

@@ -245,6 +245,7 @@ class TestRenderImplicit(TestCaseMixin, unittest.TestCase):
                     image_size=image_size,
                     blur_radius=1e-3,
                     faces_per_pixel=10,
+                    z_clip_value=None,
                     perspective_correct=False,
                 ),
             ),