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Summary:
This fixes two small issues with blending.py:softmax_rgb_blend():
  1) zfar and znear attributes are propagated from the camera settings instead of just using default settings of znear=1.0 and zfar=100.0
  2) A check is added to prevent arithmetic overflow in softmax_rgb_blend()

This is a fix in response to https://github.com/facebookresearch/pytorch3d/issues/334
where meshes rendererd using a SoftPhongShader with faces_per_pixel=1 appear black.  This only occurs when the scale of the mesh is large (vertex values > 100, where 100 is the default value of zfar).  This fix allows the caller to increase the value of cameras.zfar to match the scale of her/his mesh.

Reviewed By: nikhilaravi

Differential Revision: D23517541

fbshipit-source-id: ab8631ce9e5f2149f140b67b13eff857771b8807