a323dfae42 almost fixed subtitle tracks
disappearing when paused but it actually missed one part: the behavior
of demux_read_packet_async_until. It's a bit unintuitive, but for
subtitle streams, that function would only return the very first packet
regardless of whatever pts you pass to it. So the previous commit worked
on the very first subtitle, but not actually any of the others (oops).
This is because subtitle streams never marked as eager and thus never
actually read farther ahead. While the video is playing, this is OK, but
if we're paused and switching subtitle tracks then the stream should be
eagerly read. Luckily, the logic is already there in the function for
this. All we have to do add an extra argument to
demux_read_packet_async_until to force the stream to be read eagerly and
then it just works. Be sure to unset the eager flag when we're done.
Actually fixes the bug for real this time.
c2c157ebec