doc/writing_filters: use ffmpeg.org instead of wikimedia.org for lena image

Less likely to change, and under our control.

Thanks-to: Michael Niedermayer
ffmpeg: skip duration cliping for passthrough & drop modes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/avienc: factor frame skip code out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/avienc: write last frame duration

Fixes small part of Ticket 3052

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