Basically just bits of a9d83eac40 that was
removed and ported to here. Not really smart but it seems like flushing
buffers during seek generally leads to better behavior and avoids some
errors. The additional stream_seek call that was being performed is
definitely bogus (will never work and spams errors). So that is skipped
for bd/dvd. SEEK_FORCE is no longer needed. Instead demux_disc calls
drop_buffers on the slave demuxer (i.e. lavf).
4394ca5b6c