sub: fix subs being invisible after track reinits during pause

[Lypheo]

This fixes the issue where switching subtitles or dis- and reenabling subs during pause causes the subs to disapper until the next frame is rendered (i.e. until playback is resumed or a seek/frame step is issued). I am not confident my solution is ideal (for example I am not sure if there isn’t a place where you could just call update_subtitles at a later point in time when the demuxer is guaranteed to be caught up), but it works and has negligible performance impact.