Assimp's CalcTangentSpace deviates from de facto glTF 2.0 so we were
compensating for this with an unconditional fixup in MeshAssimp. However
the fixup should apply only when CalcTangentSpace is active, i.e. when
the model is missing tangents.
This makes it so that NormalTangentMirrorTest (has tangents) and
NormalTangentTest (needs tangents) both look reasonable.
I also noticed that MeshAssimp was inexplicably applying
aiProcess_CalcTangentSpace twice: once as a flag, and once as a
post-process. I removed the latter.
This will be fixed in the upcoming cgltf-based loader, which will
use our officially-sanctioned utility method in VertexBuffer.
See #528
