This "fixes" the new animated Fox model in the glTF conformance suite.
As per the glTF spec, we now generate per-face normals for the case
where normals are not specified in the model.
Note that true flat shading (i.e. flat interpolation) is not a
requirement in the glTF spec, since some Khronos members advocate for
WebGL 1.0 compatibility.
Fixes#2088.