Description:
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At worst imagefilledpolygon( ) doesn't render at all object composed exclusively 
of horizontal lines. No problem with vertical/diagonal lines or imagepolygon( ).

Test script:
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$image = imagecreatetruecolor(640, 480) ;

// Vertical line, drawn
$points = array(100, 100, 100, 200, 100, 300) ;
imagefilledpolygon($image, $points, 3, 0xFFFF00) ;

// Horizontal line, not drawn
$points = array(300, 200, 400, 200, 500, 200) ;
imagefilledpolygon($image, $points, 3, 0xFFFF00) ;

header('Content-Type: image/png') ;
imagepng($image) ;


Expected result:
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Two objects rendered : 1 vertical line, 1 horizontal line.

Actual result:
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One object rendered : 1 vertical line.