ffmpeg: Do not fill gap before the first decodable frame on single stream input files unless the user explicitly requests it.

Fixes different behavior to JM and probably several if not all
reference decoders.

We cannot just do this unconditionally as it would ruin AV sync in
some use cases.

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