Investigate using LUTs instead of calculating sheen weights in shader

[GSterbrant]

Description

PR #4460 adds sheen to the standard material. The glTF reference implementation uses two LUTs for the BRDFs, one to weight the underlying layers, and one to weight the reflections. Currently, the weight used to blend the layers is a constant value, which is fine, but the weight used for the environment reflections is more costly, as it's currently done in an analytical manner.