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.
At least the geometry pipeline will also use them.  Probably dynamic buffers too if it is a win (I bet yes) ( #932).   I could also see billboards using something similar for vertex compression ( #419).  3D models will also use this (but won't need the code since it will be done server-side).