Restored factor of PI to BRDFs

Added example for peppers ghost effect

Delete OBJMTLLoader.js

As the behaviour of `OBJTMLLoader` is now augmented in the `MTLLoader` and `OBJLoader`, it is no longer needed.

See PR #7524

OBJ + MTL changes

Objects are created when an "o" or a "g" are encountered. Not sure if this is correct behaviour, but the "obj_mtl" example does not work correctly without it.

This requires loading the MTL first with an instance of `MTLLoader`, and passing the resulting `materialCreator` to an instance of `OBJLoader`

WebGLRenderTarget refactoring

MMDLoader

Fix spot light cutoff.

Diffuse indirect environment light