New libav version

Fix lockup when playing some mkv files

refs #1470
New libav version

Fix issue with codec-data-private compression in matraoska

refs #1470
subs: fadein/fadeout time cannot be int16 only

refs #1470
Refactor some stuff related to subtitles.

We want to decouple the subtitle renderer from the video renderer
Move subtitle related code into src/subtitles
Enqueue subtitle packets in a separate queue

This allows us to pick subtitles that are out of sync with video stream

Also makes it possible to do negative subtitle time shifting for embedded subs

Fixes #727
Fixes #1470