Add support for generating dual-parabola environment maps

[walbourn]

Heidrich & Seidel, “View-independent Environment Maps”, Eurographics Workshop on Graphics Hardware, 1998. link

Zink, "Dual Paraboloid Mapping in the Vertex Shader".
link

Kilgard. "View Independent Environment Mapping with Dual Paraboliod Map". NVIDIA. link

Converting to/from cubemaps

[walbourn]

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

[walbourn]

Be sure to resolve this issue when this tool is ready: walbourn/directxtk12test#21

[walbourn]

Work in progress for this is in the texenvmap branch.

[walbourn]

microsoft/DirectXTK#342

[walbourn]

This seems like functionality that should be in texenvmap.

#509