Fix glTF tangent space computation, and fix default normal texture #75
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So the normals are right.
The main problem was that glTF super-material always applies a normal texture, even when the glTF doesn't have one, and the default 1x1 normal texture I was using did not have the normal pointing correctly in the +Z direction.
Also added code to compute flat normals for meshes that don't have any (as specified by the glTF spec), and to use MikkTSpace to compute the tangent and bi-tangent (again as specified by glTF) for meshes without tangents.
One downside to this PR is we're now duplicating every vertex for each face that it's in (i.e. not making use of an indexed triangle list). This could definitely be more efficient.
Fixes #3