Don't emit video until film's end if we are seeking.

Don't emit video until film's end if we are seeking.

Otherwise if we seek to the frame just before the end of the DCP
we'll call emit_video_until() with _next_video unset, causing that
method to emit video for the whole film.

The point of this emit_video_until() call is only to pad a space
between the end of the video content and the end of the film.