Add MeshRasterizerOpenGL

Summary:
Adding MeshRasterizerOpenGL, a faster alternative to MeshRasterizer. The new rasterizer follows the ideas from "Differentiable Surface Rendering via non-Differentiable Sampling".

The new rasterizer 20x faster on a 2M face mesh (try pose optimization on Nefertiti from https://www.cs.cmu.edu/~kmcrane/Projects/ModelRepository/!). The larger the mesh, the larger the speedup.

There are two main disadvantages:
* The new rasterizer works with an OpenGL backend, so requires pycuda.gl and pyopengl installed (though we avoided writing any C++ code, everything is in Python!)
* The new rasterizer is non-differentiable. However, you can still differentiate the rendering function if you use if with the new SplatterPhongShader which we recently added to PyTorch3D (see the original paper cited above).

Reviewed By: patricklabatut, jcjohnson

Differential Revision: D37698816

fbshipit-source-id: 54d120639d3cb001f096237807e54aced0acda25

tests/implicitron/test_build.py

@@ -26,8 +26,15 @@ class TestBuild(unittest.TestCase):
                     sys.modules.pop(module, None)
 
             root_dir = get_pytorch3d_dir() / "pytorch3d"
+            # Exclude opengl-related files, as Implicitron is decoupled from opengl
+            # components which will not work without adding a dep on pytorch3d_opengl.
             for module_file in root_dir.glob("**/*.py"):
-                if module_file.stem in ("__init__", "plotly_vis", "opengl_utils"):
+                if module_file.stem in (
+                    "__init__",
+                    "plotly_vis",
+                    "opengl_utils",
+                    "rasterizer_opengl",
+                ):
                     continue
                 relative_module = str(module_file.relative_to(root_dir))[:-3]
                 module = "pytorch3d." + relative_module.replace("/", ".")