Cubemap support for the Environment Texture mode

Currently, the environment texture node supports two formats: equirectangular and mirror ball. In game engines, cubemaps are a very common method of environment mapping, so it would be incredibly useful to have some sort of cubemap support for the environment texture node. The .dds format is one common way of storing cubemaps, but many programs have the ability to export/import a cubemap as a .png file as the cross-style net of a cube or as a strip of the six faces back to back