Handle the unknown

[keithgoldberg]

The current library can't handle some of the videos that my code needs to process because they also include streams that are neither video nor audio.
I've created an UnknownStream type to prevent the library from dying when these streams are present. Don't try to do anything with unknown streams since it's just a stub that allows processing to continue on the video/audio streams.

[zergon321]

Looking good to me. Could you please provide a media file with a stream that would fall into the unknown category? For me to test it out.

[keithgoldberg]

I've added a subtitle track to demo.mp4 in the analyze example.

[zergon321]

Are you sure you added the subtitles track correctly? Neither examples/analyze nor FFMpeg detect the subtitles track.

[keithgoldberg]

It looked like the subtitle track was there when I ran the following command:

`$ ffmpeg -hide_banner -i https://raw.githubusercontent.com/zergon321/reisen/23ce1f721272c2eeba1732800f3333679f5b9228/examples/analyze/demo.mp4

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'https://raw.githubusercontent.com/zergon321/reisen/23ce1f721272c2eeba1732800f3333679f5b9228/examples/analyze/demo.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf59.27.100
Duration: 00:00:25.65, start: 0.000000, bitrate: 2423 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 2206 kb/s, 29.95 fps, 30 tbr, 90k tbn, 2000k tbc (default)
Metadata:
handler_name : VideoHandler
vendor_id : [0][0][0][0]
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 209 kb/s (default)
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:2(eng): Subtitle: mov_text (tx3g / 0x67337874), 0 kb/s (default)
Metadata:
handler_name : SubtitleHandler`

[zergon321]

Oh, my bad, looked at the wrong branch. Merging.