Implement SRT output.

This is useful for push, and for bad networks (e.g. 4G).
You can in theory push to another Nageru instance, but the most
logical would either be to a Cubemap (running FFmpeg to demux,
unfortunately), or to something like YouTube, which is now working
on SRT ingest.

Note that for YouTube SRT ingest to work, someone from YouTube needs to
set a special flag on your account for now.