This gives closer results to what I've seen in papers and other renderers when
using the code to precompute albedo later (to replace MultiGGX).
It's usually a tiny difference, the only case where I've seen it matter is
in the `shader/node_group_float.blend` test - but that's a (single-scatter) GGX
closure with 0.9 roughness, so it's not too surprising. In any case, the new
result looks closer to Eevee, so that's good I guess.
Differential Revision: https://developer.blender.org/D17099
fdcb55b285