Layer diagram improvements (#189)

Improved this diagram a bit:

- expanded the glossy-diffuse slab to represent it as layer of clear-coat dielectric gloss on diffuse, as it is defined in the text. 
- indicated the transparent slabs that have embedded volume (i.e. all except the gloss).
- the number of slabs (i.e. the boxes) is now 8, matching the formal structure of the model (see bottom)
- also there are 5 transparent slabs, corresponding to the 5 slabs with a VDF in the formal structure.