Fixed Ogg Theora encoding quality

The current ffmpeg parameters for Ogg Theora leads to very low quality video. This changes uses the example command-line from http://trac.ffmpeg.org/wiki/TheoraVorbisEncodingGuide (which is within their "good range to try" suggestions).

This addresses the issue at http://pculture.freshdesk.com/support/discussions/topics/13550. (Not sure what the Bugzilla bug ID is, otherwise I'd include it here too.)