lavfi: fix broken logic in metadata handling

Signed-off-by: Paul B Mahol <onemda@gmail.com>
ffmpeg: move creation_time for ffserver setting up

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: Allow overriding ffservers input

Fixes Ticket1970

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>