NihAV: now with VP9 playback support*

Recently I’ve discovered some videos on archive.org that I’d like to watch. The problem is that they are VP8/VP9 plus Vorbis/Opus in WebMKV container which NihAV did not support. So I essentially had three options: re-encode it, use a third-party player, or implement support for it. Of course I went the fourth way—implement minimum support for the larger parts and re-encode the audio. First thing was to determine what has to be done. Since avconv can’t put VP9 into MP4, I had to implement Matroska support. It was moderately annoying but not hard. And it did not lose much respect in my eyes either (mostly because there was not much of it in the first place). Of course I ignored a lot of finer details but it seems to be good enough for my needs, I’ll eventually try it on some 10-bit H.264 plus FLAC files I have somewhere around (another codec not fit for MP4; and don’t point out at the unofficial mapping schemes—my principles say no to it). Then, audio track. That one was easy, just…