This is sort of extension of existing Use Environment option which now allows to
disable AO on the render layer basis.
Useful in cases like disabling AO for the background because it might make it
too flat and so.
Reviewers: juicyfruit, dingto, brecht
Reviewed By: brecht
Subscribers: eyecandy, venomgfx
Differential Revision: https://developer.blender.org/D1633