Merge remote-tracking branch 'origin/blender-v2.90-release' into soc-2020-production-ready-light-tree-2

Cycles: added comments to light tree code

Cycles: Cleaning up light tree code.

* removed unused update light picking code
* switched all light tree function parameters to be in the same P, V, t order
* renamed N_pick to V_pick since it is not always a normal

Cycles: never hard code -1 for t_max when calling light_sample

Cycles: Removed some unused light tree code

Cycles: Fixed small light tree volume bug

Finished bringing light tree code up to date

Merge branch 'blender-v2.90-release' into soc-2020-production-ready-light-tree-2

brought light tree code up to date

Merge branch 'blender-v2.90-release' into soc-2020-production-ready-light-tree-2

Merge commit 'e12c08e8d170b7ca40f204a5b0423c23a9fbc2c1' into gsoc-2018-many-light-sampling

Merge commit 'e12c08e8d170b7ca40f204a5b0423c23a9fbc2c1^1' into gsoc-2018-many-light-sampling

Merge branch 'master' into gsoc-2018-many-light-sampling

Cycles: light_tree_pdf() now accounts for splitting

For the MIS calculations we need to be able to calculate the
probability to sample a light using the light tree. This
did not account for splitting so if splitting was used the
probability would be wrong. This has now been fixed.

Also, if we are in PATH mode then the splitting threshold is
set to zero.

Cycles: Removed unused code

The simplified GGX code is no longer needed
with the new splitting heuristic and has
now been removed.

Cycles: Removed energy term in SAOH denominator

This should not affect the result since it is just a
scale factor to all terms in the minimization. This
term was not in eq. 2 of the paper.

Cycles: Added more comments and renamed variables

More code comments have been added to all code related to
the light tree. I also renamed all uses of "light BVH" to
use light tree instead to keep everything consistent.
Functions and variable names that used the camel case
naming convention has been changed to follow Blender's
code style. Also, unneeded includes were removed.

Cycles: Light tree: volume and MIS fixes

Now using a more reliable way of knowing if a shading point
is inside or on the boundary of a volume.

Fixed a bug in light_background_sample() that used an index
into the lights array as an index into the distribution array.

Cycles: Light tree split heuristic fix

Doubles are no longer needed in the split heuristic
calculations.

Cycles: Light tree: energy and light picking fixes

When calculating the energy for a light source I previously returned
zero energy if is_constant_emission() returned false. This has now
been changed so it uses an emission of (1,1,1) instead.

The normal that is used for light picking for the BSDF approximation
in the importance calculations now takes into consideration if the
point is on glass, a reflective or transmissive surface. The position
and normal used for light picking is now stored in the ShaderData struct.