HDR import: new Filmic tone-mapping operator, additional cleanup

Filmic Tonemapping is a very cool technique designed by John Hable for
the Uncharted video game series.  It emphasizes rich blacks and a
natural, "film-like" look over the typically oversatured "HDR-like"
appearance.  Filmic tone-mapping works very well on RAW photographs in
particular.

I've also added some normalization options for pure linear conversions.
These are helpful when dealing with scientific images (telescopes,
radiographs, etc), where having access to the full range of the data is
sometimes more important than its visual appearance.

With this, I think my tone-mapping work for 6.6 is complete.