CGeometry_ZDepth: ability to override background value

[Njen]

I would like a new attribute that overrides the background value with a user defined value.

If I am rendering an unclipped zdepth pass, I will have values above and below zero, and currently the background gets a value of zero which means it is always "in focus" when I attempt to add defocus in comp.

[Ondra]

the option is there now