Slide 15 Mod

Very very simple one-day puzzle game. It is the classic 15 slide game with the addition of a circle that must be on the centre of the image once it's completed.

The code makes it so the image is always solvable, but I'm not so sure about the getting-the-circle-in-the-centre part. I know it can be done, but I'm not sure if it is the case for every initial starting board. Any nice proofs about when can the game be solved are appreciated!

Edit: I've know found a way to trivially solve the game :(

Controls: WASD/arrow keys on web, swipe on android.

Image used is from Eric A. Anderson, at http://www.edoublea.com/photos.... Used with permission.