ffmpeg media plugin: merge muxer and demuxer tables

* The media_file_format struct has B_READABLE and B_WRITABLE flags,
allowing us to tell which formats we support decoding and encoding
* This allows application to query for the supported decoding formats.
* Also adjust some of the MIME types to match what's actually used (eg.
in HTML5 audio).

This makes html5test.com detect which formats we declare we can decode.