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.
Does `$this->config()` not work at this stage of `__construct()`?
Would prefer this as `$this->config()->default_quality` if possible.