refactoring.
change Max Ray Intensity step from 0.5 to 0.1 in Render Settings dialog.
bug fix: max ray intensity wasn't applied for secondary rays hitting the environment in PTNE engine.
fixed two major bugs in the SPPM lighting engine.

- incorrect illumination scaling when using different numbers of photons for lights and environment.
- caustics from the environment were missing when IBL was disabled.
normalize flux when viewing the photons directly (for debugging).
removed redundant method.
major knn query speed optimization in under-populated regions of highly populated trees.
reported sppm photon map size now include the photons themselves.
don't build unnecessary triangle/curve trees.
cosmetics.
removed Win32 configurations from VS solution of devkit sample.
small cleanups.
cleaned up .gitignore file.
removed redundant #include statement.
added comment.
fixed logging of focal distance when thin-lens camera's autofocus cannot find a target.
allow again empty parameters.

regression introduced in ceb4ce95e712bab60ce1b2ea57a406dd23c5a158 ("Parameter can have value between tags").
allow using colors as Disney BRDF's inputs.
added back *.opensdf files to .gitignore.
use binarymesh file format when writing orphan mesh objects to disk.