width and hight attributes (dimensions) are removed

When I use smartcrop as the only image effect the width and height attributes are gone. I tracked it down to the "image_style_transform_dimensions" function in images.module. Since both image effect don't have a "dimensions passthrough" nor a "dimensions callback" element dimensions are set to NULL.

The fix is to add a valid dimensions callback, since the image size can change.
The core callback "image_resize_dimensions" works perfectly for me.

Attached the patch to fix this.