Fuzz normal when layered on top of coat

Coat uses `geometry_coat_normal`, while other layers including fuzz use `geometry_normal`. Now that fuzz is on top of coat, this may no longer be correct.

Consider a material with a bumpy base layer, and a smooth coat layer on top that fills in the bumps. The fuzz should then have a smooth normal as well?

A solution could be to blend `geometry_normal` and `geometry_coat_normal` with `coat_weight`, and use that as the fuzz normal?

Start by tracing how `geometry_coat_normal` and `geometry_normal` are selected for coat and fuzz layers, using the layered-material example described in the issue. Compare the resulting normals for a bumpy base with a smooth coat, then establish whether the fuzz normal needs blending with `coat_weight`; done means the intended layered normal behavior is specified and covered by the relevant reference implementation or validation.

Written by the indexing model from the issue text.