Reverse the animation side for layers that are above the fixed one

[rocboronat]

Right now, if there are three layers and we fix the second one, the first one and the third one will move to the same side. The thing is that the layer that is above the fixed one should move to the reversed side.

[Sloy]

I can think of two solutions:

• Manually setting a flag to move a specific layer to the inverse side.
• Allowing negative indices for layers. By default they would start on 1. 0 means static, negative means move to the other side. This would make more sense as it would mimic the natural world, but implies an api-breaking change because right now index starts at 0.