refactorings and cleanups in the core rays vs. triangle tree leaves intersection routines.
added missing reference image.
Merge pull request #641 from est77/master

Fixed file license
Changed ContinuousTileSavingCallback to work in Blender.
Merge pull request #643 from est77/est_continuous_tile_saving_callback

Changed ContinuousTileSavingCallback to work in Blender
refactorings around the visibility flags feature.
Merge branch 'master' of github.com:appleseedhq/appleseed
cosmetics.
fixed compilation on Windows and simplified the code.
removed more inclusions of renderer/global/global.h and fixed #include statements.
added/removed #include statements as suggested by Google's IWYU.
removed unused source file.
added/removed #include statements as suggested by Google's IWYU.
minor code tweak.
code cleanups.
cosmetics.
fixed compilation by adding back a required header file.
Added BSDFSample class. Moved BSDF::Mode to the new class.
Moved BSDF::has_xxx scattering mode functions to BSDFSample.
Added BSDFSample parameter to BSDF::Sample