This moves the thickness from shadow map
approximation out of the lighting and shadowing
loop. Instead of using the thickness from
the shadow tracing from each individual light
before SSS evaluation, we precompute the
average thickness from all shadowed light.
This is then mixed with the nodetree thicknes.
## SSS Translucency
This add back SSS transmission support by using
the mentionned thickness computation, and applying the
transmission profile on it. This is then applied on top of a
flipped normal LTC computation.
Pull Request: https://projects.blender.org/blender/blender/pulls/113401
3cca8680e0