Description:
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GMagick extension does not currently support the C-API of 
GraphicsMagick FlattenImage call 
(http://www.graphicsmagick.org/api/transform.html#flattenimag
e) this call is very important when utilizing compositeimage.

Reproduce code:
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$gmagick->compositeImage($gmagickInstance, \Gmagick::COMPOSITE_DEFAULT, $x, $y);

then would need to run:
$gmagick->flattenImages();

Expected result:
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method would exist

Actual result:
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method does not exist