Refactor kernel for reflective/refractive materials

Small refactor in cuda kernels: Split the material_eval.cu in two kernels:

• reflective_kernel.cu: to handle all purely reflective materials (e.g. phong, lambert, conductors...).
• all_purpose_kernel.cu: to handle all material that may refract light (e.g. dielectric like glass...).

Common functions (e.g. MIS, NEE) are in a next-event.h header in the include folder to avoid duplication as much as possible.

Also split the microfacet.cu in two kernels:

• microfacet_conductor.cu: for microfacet-based conductors.
• microfacet_dielectric.cu: for, well, microfacet-based dielectrics...

Common functions (e.g. ndf, reflective sampling) are in a generic_microfacet.h header in the include folder to avoid duplication as much as possible.

Also, cleaned the reflective kernels to remove proxy function that were here for compatibility.

This refactor should improve performances when using reflective materials (no a priori difference for glass).