input: use DEMUX_CAN_SEEK instead of STREAM_CAN_SEEK
Some demuxers support seeking even if the underlying byte stream is not seekable (e.g. subtitles). Also some demuxers do not support seeking regardless of the underlying byte stream. So it makes more sense to check the status from the demuxer. Most demuxers just copy or forward the value from the byte stream. Note: if in doubt, a demuxer can always claim seek support, and eventually fail both DEMUX_SET_TIME and DEMUX_SET_POSITION. The seek is only used as a hint for user interfaces; it has no effects on the input thread as such.