Description:
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When using a transparency level above zero, drawing a rectangle will produce 4 dots in the corners. I think this is because the 4 lines of the rectangle are drawn and the corners will be overridden 2 times making the color on the corners darker than the rest.

Reproduce code:
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<?php
  $im = imagecreatetruecolor(100, 100);
  $bg = imagecolorallocate($im, 250, 250, 250);
  $blue = imagecolorallocatealpha($im, 150, 150, 255, 60);
  imagefilledrectangle($im, 0, 0, 100, 100, $bg);
  imagerectangle($im, 20, 20, 80, 80, $blue);
  imagepng($im);
  imagedestroy($im);
?>

Expected result:
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And grayed backgrounded image 100x100 with a blue rectangle 60x60 in the middle.

Actual result:
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Everything is ok except the corners of the rectangle are darker. In some complex examples like drawing gantt or bar charts that bug stands out even more.