grid with css rotation not handled properly

[Ystalard]

Hi Aholachek !

First, many thanks for this css grid animation tool!

I am trying to use it in order to move an element from a grid-area to another.
It works well except when the current grid is rotated : the animation is then quite weird.
You'll find a short sample here: https://github.com/Ystalard/chess-board-grid-animation

1. open sample.html on chrome
2. click on rotate button
3. click on move a piece button

It would be wonderfull if you find a solution!