CenterOn = function (rect, x, y)
{
    rect.x = x - (rect.width / 2);
    rect.y = y - (rect.height / 2);

    return rect;
}
Moves the top-left corner of a Rectangle so that its center is at the given coordinates.

@function Phaser.Geom.Rectangle.CenterOn
@since 3.0.0

@generic {Phaser.Geom.Rectangle} O - [rect,$return]

@param {Phaser.Geom.Rectangle} rect - The Rectangle to be centered.
@param {number} x - The X coordinate of the Rectangle's center.
@param {number} y - The Y coordinate of the Rectangle's center.

@return {Phaser.Geom.Rectangle} The centered rectangle.