command: sub_seek: avoid getting stuck

First, don't try to seek if the result is 0 (i.e. nothing found, or
subtitle event happens to be exactly on spot).

Second, since we never can make sure that we actually seek to the exact
subtitle PTS (seeking "snaps" to video PTS), offset the seek by 10ms.
Since most subtitle events are longer than 10ms, this should work fine.
demux_raw: set a default video size