Multiple (mostly minor) changes:

- Time is removed from the ray weight computed by the RealisticCamera
  (it's handled by the PixelSensor).
- The "fnumber" and "c" parameters have been removed from the
  PixelSensor's parameters.
- The 683 scale has been removed from both the PixelSensor imaging ratio
  and from SpectrumToPhotometric() (so a no-op, mathematically.)
- Bugfix: Medium implementations keep a separate scalar scale for their
  emission, which is now normalized like regular light sources. This will
  cause image differences.

  diffs for scenes with volumetric emission.