Z index animation fix

When I changed to Sorting Order for z indexing I introduced a bug which broke z index animation.
The current solution is a hack because you can't animate the SortingOrder property of a SpriteRenderer directly. It works while scrubbing through the timeline in the editor though.

I switched to using the SortingOrder for z indexing because having it rely on the z position of the sprite does not work with a perspective camera (it flickers).