Initial attempt at splitting for volumes.

More light tree volume tweaks

Tweeks to light tree volume code

Fixed a couple of tests with volumes with light tree disabled.

Initial volume support in light tree.

Fixed light tree construction.

There was a bug in the light tree construction that caused mesh lights
to appear as brighter than other lamp lights.

Fixed CUDA support for the light tree.

Users can now use the light tree, with splitting, using CUDA.

The problem was that a few of the light tree functions used recursion
which meant the CUDA compiler couldn't predict how much stack space
to use.

Fixed motion blur with light tree enabled.

Decoupled splitting from branched path integrator.

Changed calulation of theta_u in tree importance.

This reduces artifacts when using the light tree without splitting.

Before the theta_u was calclulated using a bounding sphere to
approximate a bounding box. This sometimes resulted in the cone
containng the bounding box much, much too big. Now theta_u is found
more carefully so that it is the smallest angle such that the cone
completely contains the bounding box.

Fixed how last not-transparent bounce is found.
Was using emission_sd if most recent bounce was transparent, but this
is incorrect. Now it uses the ray_N PathState for normal and
recalculates the point.

Fixed using wrong angle in light tree importance

Fixed most render tests with light tree off.

Fixed light tree Primitive being struct and class.

Fixed inconsisancy causing calling direct_emission twice from merge.

Removed VLA's leftover from the 2018 GSoC project.

Fixed bug when not using light tree from merge.

Made Light Tree an experimental feature

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

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