generating face normals may leave some extra data around

[tfili]

It looks like the original position data may not be cleared.

Also the test generates face normals if they do not exist doesn't actually test anything, because smooth normals are generated in a previous test. We need to set these tests up to use fresh copies of the models.

[likangning93]

Also the test `generates face normals if they do not exist doesn't actually test anything

Woah, looks like there's errors in this code path that must have slipped by because of this: https://github.com/AnalyticalGraphicsInc/gltf-pipeline/blob/master/lib/generateNormals.js#L99