Allow changing reflectance models for standard material

Running standard material is too heavy for some of the lower end HW. One thing that takes up a lot of cycles is Oren Nayar reflectance model. We could either optimize it automatically down to blinn-phong or lambertian on lower end models or allow user to select which reflectance model they want to use. As the standard material now it is not usable on lower end and you are forced to create custom material. (there is relation to the node based shading here).