avfilter/vf_idet: Fixing idet for single-frame inputs.

Handle single frame inputs similar to yadif (e.g., https://github.com/FFmpeg/FFmpeg/commit/0f9f24c9cfd291c7ece4d3bad64fdf06d107168a and https://github.com/FFmpeg/FFmpeg/commit/681e008d06d2241d50abe6316c908a184ddc5942)

Example:
  ffmpeg -r 1 -t 1 -i fate-suite/ffmpeg-synthetic/vsynth1/%02d.pgm  -vf idet,showinfo -f null -y /dev/null

Previously:
  Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
  [Parsed_idet_0 @ 0x36389d0] Repeated Fields: Neither:     0 Top:     0 Bottom:     0

After patch:
  [Parsed_showinfo_1 @ 0x1909810] n:0 pts:0 pts_time:0 pos:-1 fmt:gray sar:0/1 s:352x432 ...
  [Parsed_idet_0 @ 0x18f9bb0] Repeated Fields: Neither:     1 Top:     0 Bottom:     0

Fate looks good.

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>