Allow $crop in add_image_size() to receive crop anchors (top, left, right, bottom, center).

This also applies to set_post_thumbnail_size() and image_resize_dimensions().

props bradt, wonderboymusic, DH-Shredder.
fixes #19393.


git-svn-id: https://develop.svn.wordpress.org/trunk@27472 602fd350-edb4-49c9-b593-d223f7449a82