Fix oct-encoded normal upsampling. 

Cannot linearly interpolate oct encoded normals in all cases, due to how the octrahedron is unwrapped into a unit square.
I think this wording is a little klunky and we we usually make these link tags.  How about?

`Encodes a normalized vector into 2 bytes following the 'oct' encoding as described in {@link http://jcgt.org/published/0003/02/01/|A Survey of Efficient Representations of Independent Unit Vectors}.`