Tweak color of unlit areas

[fr0]

First, thanks for this library. It's awesome and it's solved so many problems for me.

Is it possible to have the unlit areas be anything other than black? In my game, I want them to be "foggy", and the spotlight from the player should cut through that fog to reveal the play area. (I'd probably eventually want to render that fog with a shader).

I originally thought that using a non-black AmbientColor would work but that also changes the color of the lit areas, which I don't want.