Shading basis non-orthogonal on analytical sphere test scene

[dictoon]

Test scene

analytic lights/06 - sphere - analytic.appleseed

fails with an assertion in debug because the shading basis we try to construct in Intersector::make_procedural_surface_shading_point() is not orthogonal:

shading_point.m_shading_basis = Basis3d(
    normal,
    normalize(dpdu),
    normalize(dpdv));