Currently the specular/glossy component always uses a weight of 1 even if it
just contributes a tiny amount to the result.
This patch adds an estimate for the weight that makes the sampling weight
more reasonable, which can drastically improve noise in some cases.
Also, this is important for denoising feature pass generation, since it uses
sampling weights of closures in its specular/diffuse heuristic. With the
current weighting, even setting Specular to 0.001 will cause Cycles to
wait for the next bounce and therefore ignore the texture while denoising.