lavf/concatdec: support seeking.
lavfi/concat: fix silence duration computation.

Inside send_silence(), delta_ts was used inconsistently:
sometimes as the new value, sometimes as the old value.

Consistently use it as the new value, and add an argument
to know the last segment duration.
lavu/opt: add AV_OPT_TYPE_DURATION.
lavfi: add sine audio source.
ffmpeg: fix freeing of sub2video frame.
lavf/vobsub: free index pseudo-packet.
lavfi: add common code to handle options parsing.
lavfi/avf_concat: use standard options parsing.
lavfi/vf_tile: use standard options parsing.
lavfi/af_volume: use standard options parsing.
lavfi/vf_crop: use standard options parsing.
lavfi/vf_decimate: use standard options parsing.
lavfi/vf_delogo: use standard options parsing.
lavi/vf_drawbox: use standard options parsing.
lavfi/vf_fade: use standard options parsing.
lavfi/vf_fps: use standard options parsing.
lavfi/vf_pad: use standard options parsing.
lavfi/vf_transpose: use standard options parsing.
lavfi/vf_unsharp: use standard options parsing.
lavfi/vf_yadif: use standard options parsing.