(bug 16451) Fix application of scaling limits for animated GIFs.

(bug 16451) Fix application of scaling limits for animated GIFs.
* Creates a new media handler for GIF files, and extracts metadata such as duration, whether or not the GIF is looped, and the number of frames.
* Uses the extracted metadata for the long file description.
* Considers number of frames in the calculation of image area (multiplies by width and height to get the "time-area", or so to speak).

After this patch is deployed, the work-around to disable GIF scaling on Wikimedia sites can be eliminated.