Add support for generating octahedron environment maps

[walbourn]

Engelhardt & Dachsbacher, "Octahedron Environment Maps", Visualization Research Center, University of Stuttgart. link

Cigolle et al, "A Survey of Efficient Representations for Independent Unit Vectors", Journal of Computer Graphics Techniques Vol. 3, No. 2, 2014. link

Lottes. "Fetching From Cubes and Octahedrons". AMD. link

[walbourn]

In other words, given six faces of a cubemap, generate a octahedron environment map from it.

[walbourn]

This seems like functionality that should be in texenvmap.

#509