demux: fix erroneous condition in lazy_stream_needs_wait

Yeah another try at this. So when inspecting lazy_stream_needs_wait, I
realized it had a curious !ds->reader_head condition. Actually, this is
what is messing everything up. This was originally added in
cf2b7a4997 for showing large negative sub
delay values correctly. It worked because the packet will eventually be
discarded during playback causing ds->reader_head not exist and thus the
next one will correctly be read ahead as needed.

But for the "switching subtitle tracks while paused" case, this is
actually bad. As the stream is read, eventually you'll find a packet and
set the reader_head. But it's not going to be the correct packet (unless
you're looking for the very first one), so you need to read more. This
won't happen because of the !ds->reader_head check and unlike the sub
delay case, nothing will eventually discard that packet since playback
isn't occuring. So read_packet exits earlier than it should and isn't
tried again, so the subtitle that you want won't show since the
returned packet has the wrong pts. All that needs to be done here is to
delete this one condition. There's already checks in place to make sure
that it's not read past the desired timestamp and for the sub delay case
(the only other time this logic is used), it makes no difference since
you won't read past the specified pts in the first place.

demux/demux.c

@@ -2153,7 +2153,7 @@ static bool lazy_stream_needs_wait(struct demux_stream *ds)
     struct demux_internal *in = ds->in;
     // Attempt to read until force_read_until was reached, or reading has
     // stopped for some reason (true EOF, queue overflow).
-    return !ds->eager && !ds->reader_head && !in->back_demuxing &&
+    return !ds->eager && !in->back_demuxing &&
            !in->eof && ds->force_read_until != MP_NOPTS_VALUE &&
            (in->demux_ts == MP_NOPTS_VALUE ||
             in->demux_ts <= ds->force_read_until);