subtitles: improve support for libavformat demuxed subtitles

Make demux_lavf not error out if no video or audio track is present.
This allows opening subtitle files with the demuxer.

Improve the test whether subtitles read from demuxers must do explicit
packet reads. (I'm not sure whether always doing these reads could have
bad effects, such as reading too many audio and video packets at once,
so be conservative.)

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