Description:
------------
When drawing a polygon with the default thickness everything is fine; but when setting the thickness of the line > 1, I get strange results. It looks as if certain lines of the polygons are not being drawn with the correct thickness.

Reproduce code:
---------------
function getPolygon($width, $border_width, $sides)
{
    $xcen = $width; $ycen = $width; $incr = 360/$sides;
    $angle = 180; $k = 0; $max_height = 0;
    for($j = 0; $j < $sides; $j++)
    {
        $points[$k] = $xcen + $width * sin(deg2rad($angle)) + $border_width;
        $k++;
        $points[$k] = $ycen + $width * cos(deg2rad($angle)) + $border_width;
        $max_height = max($max_height, $points[$k]);
        $k++;
        $angle += $incr;
    }
    $img = imagecreatetruecolor(2*$width+10, $max_height+$border_width+10);
    $bg = imagecolorallocate($img,255,255,255);
    ImageFill($img, 0, 0, $bg);
    imagecolorallocate($img, 255, 255, 255);
    $black = imagecolorallocate($img,0,0,0);
    imagesetthickness($img,$border_width);
    imagepolygon($img,$points,$sides,$black);
    header("Content-type: image/gif"); imagegif ($img); imagedestroy($img);
}
getPolygon(100, 5, 9);

Expected result:
----------------
For 9 sides, it should be like http://www.gabber.nl/cp/geometry_1px.gif with a thicker line. 

Actual result:
--------------
The output is http://www.gabber.nl/cp/geometry_5px.gif . This happens for most values of sides.