Shading normal & Normal/Bump Map & Acceleration

[Enigmatisms]

Shading normal is finally supported, resulting in a much smoother shading. Besides this:

• Bump map / normal map can be applied to primitives to manipulate the surface appearance.
• Volumetric path tracer now supports MIS, the firefly effect in vpt is suppressed.
• Loading from wavefront obj file and BVH construction are accelerated GREATLY (over 1000x), the scene with more than 100k primitives can now be loaded into Taichi instantly.
• Rendering is accelerated (slightly, about 2x) for CUDA backend
• Support up to Taichi version 1.6 (from 1.4.0 to 1.6.0, all these versions are supported)

Newly rendered scene: venus can be found in the AdaptiveGallery. The three-bunnies scene is also re-rendered with vertex normal support (and miscellaneous stuff like proper total-reflection)

[raven-sera]

Well done!

[Enigmatisms]

Merge PR 19 and releasing version 1.4.0