Fix bug in transformation of dndu and dndv in Triangle::GetShadingGeometry.

These should be transformed by the passed-in obj2world transformation, not
by *ObjectToWorld stored in the Shape.

Reported by Yunjian Ding.
Add missing cosine term in IGI light path tracing.

Reported by bachi722@gmail.com.
Add tools/obj2pbrt (convert Wavefront OBJ files to pbrt).

Based on Syoyo Fujita's tinyobjloader: https://github.com/syoyo/tinyobjloader