[Bug]brdfSGMus for specular light should be divided by 2

[Tianhang-Cheng]

In InvRender (https://github.com/zju3dv/InvRender/blob/f1dd52ac6a20eddc9a9908a754e619e592866e16/code/model/sg_render.py#L309), Here inv_roughness_pow4 = 2. / (roughness * roughness * roughness * roughness) # [dots_shape, 1] mu_val = (inv_roughness_pow4 / np.pi).expand(dots_shape + [3]) # [dots_shape, 1] ---> [dots_shape, 3]

But according to PhySG (https://github.com/Kai-46/PhySG/blob/030d590bc007377007c65046c8f70ad149cfb371/code/model/sg_render.py#L173) and its supplementary material (https://kai-46.github.io/PhySG-website/files/supp.pdf), mu_val should be 1 / (pi* roughness ^ 4) rather than 2 / (pi* roughness ^ 4)