GetNumberArray = function (polygon, output)
{
    if (output === undefined) { output = []; }

    for (var i = 0; i < polygon.points.length; i++)
    {
        output.push(polygon.points[i].x);
        output.push(polygon.points[i].y);
    }

    return output;
}
Stores all of the points of a Polygon into a flat array of numbers following the sequence [ x,y, x,y, x,y ],
i.e. each point of the Polygon, in the order it's defined, corresponds to two elements of the resultant
array for the point's X and Y coordinate.

@function Phaser.Geom.Polygon.GetNumberArray
@since 3.0.0

@generic {number[]} O - [output,$return]

@param {Phaser.Geom.Polygon} polygon - The Polygon whose points to export.
@param {(array|number[])} [output] - An array to which the points' coordinates should be appended.

@return {(array|number[])} The modified `output` array, or a new array if none was given.