This PR fixes the bug with the squared metallic term in the reference BRDF implementation in Appendix B by rewriting the pseudocode according to the proposal of @lisyarus. In addition, all extensions referencing the code are updated.

Besides, I removed a few inconsistencies in the extensions:

• Snake case is now consistently used for values in the implementation, and camelCase for public parameters.
• BRDFs are now called xxx_brdf instead of a mixture between f_xxx and xxx_brdf.
• The clearcoat extension now differentiates more clearly between clearcoat normal (Nc) and material normal (N)
• The visibility term is now referred to as $\mathcal{V}$ to distinguish it from the view direction $V$.
• The specular extension now uses the same structure for factor * color multiplications as sheen.