Uses a light tree to more effectively sample scenes with many lights. This can
significantly reduce noise, at the cost of a somewhat longer render time per
sample.
Light tree sampling is enabled by default. It can be disabled in the Sampling >
Lights panel. Scenes using light clamping or ray visibility tricks may render
different as these are biased techniques that depend on the sampling strategy.
The implementation is currently disabled on AMD HIP. This is planned to be fixed
before the release.
Implementation by Jeffrey Liu, Weizhen Huang, Alaska and Brecht Van Lommel.
Ref T77889
* Split light types into own files, move light type specific code from
light tree and MNEE.
* Move flat light distribution code into own kernel file and host side
building function, in preparation of light tree addition. Add light/sample.h
as main entry point to kernel light sampling.
* Better separate calculation of pdf for selecting a light, and pdf for
sampling a point on the light. The selection pdf is now also stored in
LightSampling for MNEE to correctly recalculate the full pdf when the
shading position changes but the point on the light remains fixed.
* Improvement to kernel light storage, using packed_float3, better variable
names, etc.
Includes contributions by Brecht Van Lommel and Weizhen Huang.
Ref T77889
* Rename "texture" to "data array". This has not used textures for a long time,
there are just global memory arrays now. (On old CUDA GPUs there was a cache
for textures but not global memory, so we used to put all data in textures.)
* For CUDA and HIP, put globals in KernelParams struct like other devices.
* Drop __ prefix for data array names, no possibility for naming conflict now that
these are in a struct.
* Replace license text in headers with SPDX identifiers.
* Remove specific license info from outdated readme.txt, instead leave details
to the source files.
* Add list of SPDX license identifiers used, and corresponding license texts.
* Update copyright dates while we're at it.
Ref D14069, T95597
Remove prefix of filenames that is the same as the folder name. This used
to help when #includes were using individual files, but now they are always
relative to the cycles root directory and so the prefixes are redundant.
For patches and branches, git merge and rebase should be able to detect the
renames and move over code to the right file.
