Refactor and simplify new microfacet models

- Refactor common code between isotropic and anisotropic cases.
- Corrected some method names.
- Removed MaskingShadowing template template parameters.
- Made utility methods in MDF static.

Microfacet2 tests pass.
Optimization:

     const T cos_phi_2_ax_2 = square(h.x / (sin_theta * alpha_x));

Same thing in many other places.