Blender Git Loki

- decided to use a new variable for the new exposure option, instead of
re-using old one. New one = 'exp'.
- at first I used the old 'exposure' value, and just mapped it to 0. this
causes a problem with upward compatibility, old blenders then render a
black picture. is too confusing!
- warning; exposure values saved with commit of last week will get lost.