Scattering mode for standard hair

A scattering option scattering_mode has been added to the standard hair shader. Use the approximate mode to get the previous behavior. The accurate mode can be noisier, and so takes about 10–30% longer to render, but delivers more realistic results, especially in close-ups, by simulating each hair as a cylinder instead of flat surface. The adaptive mode uses the accurate mode for close-up hairs and the approximate for far-away hairs, automatically balancing quality and performance.