Image Magick PaddedResize fix

This fixes the padded resize problem mentioned in #3333

Note that Image_Backend doesn't have any cross-backend unit tests.

In this answer, someone suggests stripping out image metadata, and doing a MD5 checksum on the bits.
http://stackoverflow.com/questions/7149747/need-help-understanding-difference-in-raw-image-binary-data-for-phpunit-test
Not sure how well this will work, as there are surely slight differences with image quality etc across backends.
I'm going to guess that it wont, considering this line is in consistent with the Image_Backend interface definition of the function, and also the same definition within GD.php