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Summary:
There was a bug when `z_clip_value` is not None but there are no faces which are actually visible in the image due to culling.  In `rasterize_meshes.py` a function `convert_clipped_rasterization_to_original_faces` is called to convert the clipped face indices etc back to the unclipped versions, but the case where there is no clipping was not handled correctly.

Fixes Github Issue #632

Reviewed By: bottler

Differential Revision: D29116150

fbshipit-source-id: fae82a0b4848c84b3ed7c7b04ef5c9848352cf5c