Description:
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When using opacity, it seems to work ok, until I use roundCorners, once I do this, what should be transparent, goes black

Reproduce code:
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<?php
	$im = new Imagick();
	$im->newImage(400,400,new ImagickPixel('green'), 'png');
	$im->roundCorners(50,50);
	$im->setImageOpacity(0.5);
	header('Content-Type: image/' . $im->getImageFormat());
	echo $im->getImageBlob();

Expected result:
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Outside of the green section should be transparent

Actual result:
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Outside of the green has gone black