tweaked formatting.
tweaked comment.
added infrastructure to support aborting rendering during scene preparation, and improved aborting mechanism.

at this point, the following operations can be aborted:
  - preparation of scene entities (calling of on_frame_begin() handlers)
  - construction of latitude-longitude importance maps

the following operations cannot yet be aborted:
  - geometry collection
  - building of acceleration structures
  - construction of intersection filters
in appleseed.studio, only compute the final average luminance if the print_final_average_luminance setting is set to true (false by default).
updated appleseed.studio's settings file.