Add 3D layer offset as style property

[Bauumm]

Originally I made this to shift 3D layers up by one for 1.92 parity (for the level converter), but it turned out that quite a lot of fun effects can be done with this, so I also added the ability to render 3D layers above the main layer and a way to mirror the alpha falloff as well as the option to render the main layer on top of all 3D layers.