Shaders: renaming more functions 2

[WestLangley]

To be a bit less verbose...

Part A

BRDF_Diffuse_Lambert() -> BRDF_Lambert()

BRDF_Specular_BlinnPhong() -> BRDF_BlinnPhong()

BRDF_Specular_GGX() -> BRDF_GGX()

BRDF_Specular_Sheen() -> BRDF_Sheen()

Part B

BRDF_Specular_Multiscattering_Environment() -> computeMultiscattering()

The above function is not a brdf. It computes two reflectance quantities. So renaming it to computeMultiscattering for now.

BRDF_Specular_GGX_Environment -> EnvironmentBRDF

This is not a brdf either. It returns a reflectance, but "EnvironmentBRDF" is a commonly-used term, so using that for now.

[WestLangley]

@mrdoob Part A is subjective, and my preference. If you don't like it, I'll revert Part A.

[mrdoob]

I like the short names 👍

[mrdoob]

Thanks!