IMCE should call Drupal's image_get_info() rather than getimagesize() directly

At Pantheon, we use extensive caching around image_get_info() to avoid network file system traffic. Calling getimagesize() bypasses this cache with bad consequences for users browsing many images through IMCE.

For quick reference, this is the call we'd like it to use:
http://api.drupal.org/api/drupal/includes%21image.inc/function/image_get...

Making excessive calls to getimagesize() and related file system functions on network mounts is a known problem that even has its own module for certain cases: http://drupal.org/project/imageinfo_cache

Pantheon will award a bounty of $100 for getting this simple fix into stable releases of IMCE for Drupal 6 and 7 by November 8, 2012.

The included patch is for Drupal 7.