Smoothing groups

[Wavefront .obj files](http://paulbourke.net/dataformats/obj/) have the concept of "smoothing groups," which basically just means "interpolate the normals of these faces so the group of faces looks like a smooth surface."

pbr reads the basic .obj format but doesn't currently respect smoothing groups, so models that rely on smoothing groups end up looking like this:


The issue names no files, tests, or entry points. Start by locating pbr’s basic Wavefront .obj reader and its existing normal-handling path, then determine how to verify that smoothing-group faces are interpolated and models using them render smoothly.

Written by the indexing model from the issue text.