Improved handling of thumbnail generation for image types
* Now honors the width/height in the options
* Removed some unnecessary complexity, switched to "image_downsize" instead of "wp_get_attachment_image_src," which was just calling "image_downsize" along with other work that wasn't needed

git-svn-id: https://plugins.svn.wordpress.org/document-gallery/trunk@888563 b8457f37-d9ea-0310-8a92-e5e31aec5664