fate: add rawdiff and use it for subtitles

We want subtitles tests to match exactly line endings and trailing
characters.
fate: fix some subtitles tests dependencies
avformat/assenc: support remuxing old V4 ASS form (SSA)
avcodec/ass: output missing fields in AVSubtitles and output files

Fixes Ticket #3207
avcodec/ass_split: extend recognized fields in ASS splitter

This simplifies the logic for the fix in the next commit.
avcodec/ass_split: assume default order when "Format:" line is not found

Fixes the decoding of the ASS stream in a mkv from a fansub release from
Coalgirls.
avcodec/ass: add FFmpeg signature