Description:
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There's no reason why imagecopyresized shouldn't accept and properly handle a 
negative width and/or height as source or target dimentions.

It's perfectly natural to copy a rectanguar region of an image while applying it 
a negative scale so as to flip it. It's totally ridiculous that you have to copy, 
flip and then copy it again resized. Especially considering  that prior to 5.5 
imageflip doesn't even exist.

At the very least negative dimensions should be supported on either source or 
target, though the normal way would be to support both.

Test script:
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imagecopyresized($img,$img,0,$h=imagesy($img),0,0,$w=imagesx($img),-$h,$w,$h);

 OR	

imagecopyresized($img,$img,0,0,0,$h=imagesy($img),$w=imagesx($img),$h,$w,-$h);

Expected result:
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image should be flipped vertically

Actual result:
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An error is issued:
imagecopyresized(): Invalid image dimensions