Cubemap generation mode for TextureImporter.
          Cubemap textures can be generated from 2D texture using one of these modes.
See Also: TextureImporter.generateCubemap.
| Spheremap | Generate cubemap from spheremap texture. | 
| Cylindrical | Generate cubemap from cylindrical texture. | 
| FullCubemap | Generate cubemap from vertical or horizontal cross texture. | 
| AutoCubemap | Automatically determine type of cubemap generation from the source image. |