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

pytorch3d/renderer/utils.py

@@ -8,7 +8,7 @@
 import copy
 import inspect
 import warnings
-from typing import Any, Optional, Tuple, Union
+from typing import Any, List, Optional, Tuple, Union
 
 import numpy as np
 import torch
@@ -432,3 +432,27 @@ def ndc_to_grid_sample_coords(
     else:
         xy_grid_sample[..., 0] *= aspect
     return xy_grid_sample
+
+
+def parse_image_size(
+    image_size: Union[List[int], Tuple[int, int], int]
+) -> Tuple[int, int]:
+    """
+    Args:
+        image_size: A single int (for square images) or a tuple/list of two ints.
+
+    Returns:
+        A tuple of two ints.
+
+    Throws:
+        ValueError if got more than two ints, any negative numbers or non-ints.
+    """
+    if not isinstance(image_size, (tuple, list)):
+        return (image_size, image_size)
+    if len(image_size) != 2:
+        raise ValueError("Image size can only be a tuple/list of (H, W)")
+    if not all(i > 0 for i in image_size):
+        raise ValueError("Image sizes must be greater than 0; got %d, %d" % image_size)
+    if not all(type(i) == int for i in image_size):
+        raise ValueError("Image sizes must be integers; got %f, %f" % image_size)
+    return tuple(image_size)