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.
Will this break existing use of the `$backgroundColor` argument that includes a `#` character?