constructor( renderer ) {

			_renderer = renderer;

			_renderer.compile( _mesh, _flatCamera );

		}
This class generates custom mipmaps for a roughness map by encoding the lost variation in the
normal map mip levels as increased roughness in the corresponding roughness mip levels. This
helps with rendering accuracy for MeshStandardMaterial, and also helps with anti-aliasing when
using PMREM. If the normal map is larger than the roughness map, the roughness map will be
enlarged to match the dimensions of the normal map.