Add support for retina_images.module in Caption Filter

Despite its low usage, in this developer's opinion, the Retina Images module is poised to become The Drupal Standard for retinizing images in the near future, especially given that Picture module is going in core with Drupal 8.

Retina Images introduces problems with Caption Filter's _caption_filter_get_width() func. Because retina images are twice the width of their display dimensions, Caption Filter's inline width value is twice what it should actually be.

One potential solution is to load the Image Style preset and see if the retina images module is enabled for that preset. If it is, divide the getimagesize() value by half.