lavf/gif: correct the delay after the first frame.

To define accurately the delay between two frames, it is necessary to
have both available. Before this commit, the first frame had a delay of
0; while in practice the problem is not visible in most situation, it is
problematic with low frame rate and large scene change.

This commit notably fixes output generated with commands such as:

  ffmpeg -i big_buck_bunny_1080p_h264.mov
     -vf "select='gt(scene,0.4)',scale=320:-1,setpts=N/TB"
     -frames:v 5 -y out.gif

Also, to avoid odd loop delays, the N-1 delay is duplicated for the last
frame.
