Merge pull request #651 from est77/master

Renamed BSDFSample::get_normal to get_shading_normal for consistency
Fix autocomplete error
Microfacet2 refactors and cleanup.
Refactored Beckmann MDF
Refactored GGX
More MDF2 refactors
Minor changes to eval_D methods
Merge pull request #652 from est77/est_MDF2

Refactor and simplify new microfacet models
PR comments changes