Fix Z-index on tiles that don't snap to grid

[Toldoven]

When you drop a tile and you already have tiles on the bottom and on the right – the new tile will render behind them. Which will make it hard to get this tile out of there if you placed it in the wrong spot

We can fix this by changing the Z-index of the tiles that are placed in the wrong sport. However Godot 3.5 simply doesn't have Z-index property so the fix to this problem is not trivial. It's a thing in Godot 4 though.