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).