avformat/assdec: output ASS packets

After this the order from the original file is stored through readorder
when doing ffmpeg -i input.ass -c copy output.mkv.

And now that the ASS muxer honors the ReadOrder, extracting the ass back
(without transcoding) restores the original order.
avformat/assdec: make sure "Comments:" events end up in the header
avformat/assdec: reindent after previous commit
avformat/assdec: add myself to Copyright
avformat/assenc: mux an event header if none are found in extradata
avformat/assdec: make sure to free allocated data in case of error
avcodec/microdvddec: remove old compat code
avcodec/subviewerdec: remove old compat code