This is a simple trick that does *not* have a huge performance impact but
does work pretty well. It just modifies the Fresnel term to account for
the multibounce energy loss (coloration).
However this makes the shader variations count double. To avoid this we
use a uniform and pass the multiscatter use flag inside the sign of f90.
This is a bit hacky but avoids many code duplication.
This uses the simplification proposed by McAuley in
A Journey Through Implementing Multiscattering BRDFs and Area Lights
This does not handle area light differently than the IBL case but that's
already an issue in current implementation.
This is related to T68460.