ffmpeg: When streamcopying, only add the input seek position when copying timestamps.

Using -ss as an input option shifts timestamps down by the seek, so it
doesn't have to be added to the recording time when checking whether to
stop.

Fixes #977

Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
