Use of IMagick#extentImage results in images with incorrect dimensions

This occurs both for images which are already the correct size, and end up being scaled to 511x512, 511x511, etc., and images which *should* be padded, but end up with a similarly incorrect final size.

Further, the problem can be shown to lie with extentImage, and not the methods that compute the padding values. To see this, start with a 512x512 image, and call $image->extentImage(512,512,0,0). The result is a 511x512 image in some instances, even though it should be an (unmodified) 512x512 image.