[Bf-codereview] Brightness/Contrast Node for Cycles (issue 5564044)

[dfelinto at gmail.com]

Brecht, I added the float postfix everywhere (OSL included).

I have a feeling we may be able to get another bright contrast algorithm
later. This one came from Blender which came from opencv. It works fine
but I wonder what gimp is doing for example.

http://codereview.appspot.com/5564044/