use of std::bind was copying the underlying random number generator before
use, resulting in the exact same sequence getting reused for, ie, every
AO calculation within a given tile-group

- forward-declare some types as struct instead of class (avoids msvc
  compiler warning)
- comment typo fix

Adds more detailed and precise explanations of expected behavior for
UsdLux lights to UsdLux docs.

This modifies the hdEmbree example plugin to do direct lighting so as to
provide a reference implementation of the expected behaviour for UsdLux.

If no lights are present in the stage, the old ambient occlusion path
will be used.

A few small utility functions from pbrt-v4

These are taken unaltered from cycles, v4.1.1

- https://projects.blender.org/blender/cycles/src/tag/v4.1.1/src/util/ies.h
- https://projects.blender.org/blender/cycles/src/tag/v4.1.1/src/util/ies.cpp

These are not yet built / used, but merely introduce the files for use
in the codebase later.

default

...since, unlike Cycles, we intend to work in candela

…ntegration

…pdate