**Patch for**: T17899

Currently the tangent shading (TANGENT_V) produces disappointing results which are quite unphysical and show stupid artifacts like stairs. (See tangent_shading_original.png)

Many people have also reported the same problem:

http://projects.blender.org/tracker/index.php?func=detail&aid=17899&group_id=9&atid=264

http://blenderartists.org/forum/showthread.php?112347-tangent-shader-rayshadow-artifacts

http://blenderartists.org/forum/showthread.php?136684-strange-pixilated-effect-with-tangent-shading

http://blenderartists.org/forum/showthread.php?170493-Specular-Lightning-issue-with-Tangent-V

http://blenderartists.org/forum/showthread.php?206292-Problem-with-Tangent-V-Shading-Shadows

So I tried improving it as follows:

1. Don't show the artifacts

2. Use proper method for determining fake face normal

3. Consider self-shadowing and self-occlusion due to microfacets

4. Add 'Anisotropy' slider to material panel (anisotropy_ui.png)

I noticed the expressions to determine the fake face normal are mathematically incorrect, then I replaced them with exact one. The calculations of 3 and 4 are not so physically accurate, but we should be able to obtain sufficiently nice results. (See sample pictures)

Note that the self-shadowing/occlusion doesn't work for toon shaders.