[Bf-cycles] Shading tangents patch (for the ward anisotropic closure)

[Mike Farnsworth]

Here is a patch that re-enables the Ward anisotropic shading node, and
builds proper smoothed tangents on meshes for shading.  It only builds
and uploads the data as a node requests it, to minimize the memory
hit.  It computes tangents per face-vertex (corner), as it will
respect discontinuous UVs as they are assigned by the artist.

In the case the UVs are missing, it currently tends to just render
black.  It would be nice if it fell back to something more reasonable
(like faceted dPdu), but I couldn't easily figure out how to tell the
node in the kernel that no loadable tangents were available.  So, it
is currently a hard requirement for using the anisotropic shading
closure that UVs must be laid out on the mesh first.

-Mike
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