MergeRect = function (target, source)
{
    var minX = Math.min(target.x, source.x);
    var maxX = Math.max(target.right, source.right);

    target.x = minX;
    target.width = maxX - minX;

    var minY = Math.min(target.y, source.y);
    var maxY = Math.max(target.bottom, source.bottom);

    target.y = minY;
    target.height = maxY - minY;

    return target;
}
Merges the source rectangle into the target rectangle and returns the target.
Neither rectangle should have a negative width or height.

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

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

@param {Phaser.Geom.Rectangle} target - Target rectangle. Will be modified to include source rectangle.
@param {Phaser.Geom.Rectangle} source - Rectangle that will be merged into target rectangle.

@return {Phaser.Geom.Rectangle} Modified target rectangle that contains source rectangle.