/// <summary>
/// Internal funciton to invoke the actual movement of a tile.
/// Assumes position is valid, tile exists, etc.
/// Used by SwapTile & refillBoard
/// </summary>
private void MoveTile(Tile tile, PairInt from, PairInt delta,
bool isBackground=false)
{
tile.MoveBy(from, delta, isBackground);
PairInt target = from + delta;
board[target.x, target.y] = tile;
tile.UpdateName(target);
}