This adds a new image_resize() function, plus ancillary functions in wp-includes/media.php.
image_resize() resembles wp_create_thumbnail() and wp_crop_image(). It's more general than wp_create_thumbnail(), and supports a cropping mode for square thumbnails.
The new function isn't called anywhere yet but it's unit tested. More patches to come will hook it up to thumbnail generation and other stuff.
Unit tests are in http://svn.automattic.com/wordpress-tests/wp-testcase/test_image.php, particularly TestImageResizeDimensions? and TestImageResize?.
