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: 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.

Cycles: Light tree instancing and energy fixes

Instancing for mesh lights now works with the light tree.
The conversion from emission to luminance is now using
linear_rgb_to_gray() instead.

Cycles: WIP: Volumes are not using the light tree.

If a shading point is inside or on the boundary of a volume
then it will use the old sampling method until we have
properly implemented the volume parts of the paper.

Also, the PDF picking probability for volumes have now been
updated properly since the PDF restructure.

Tried to provide the correct shading point and normal to
the MIS calculations. This part needs more work.

Cycles: Light tree optimization

The importance metric calculations now uses the
fast_cos() and fast_acos() functions. This gave
a 1.4x speedup on one of the test scenes.

Cycles: Light tree related bug fixes

- Fixed bug in triangle_light_pdf_area() from PDF refactor
- Early exit if picking prob is zero in tree traversal
- The background index is now an index into the lights
array instead of the distribution array.