avfilter/vf_idet: Fixes issue with idet not flushing last frame.

Uses a similar approach as vf_yadif to flush the last frame in idet.

Quick test with 50 frames from vsynth1:
./ffmpeg.old -i fate-suite/ffmpeg-synthetic/vsynth1/%02d.pgm -vf idet -f mp4 -y /dev/null 2>&1  | grep Multi
 (gives) [Parsed_idet_0 @ 0x261ebb0] Multi frame detection: TFF:0 BFF:0 Progressive:48 Undetermined:1

./ffmpeg -i fate-suite/ffmpeg-synthetic/vsynth1/%02d.pgm -vf idet -f mp4 -y /dev/null 2>&1  | grep Multi
 (gives) [Parsed_idet_0 @ 0x35a0bb0] Multi frame detection: TFF:0 BFF:0 Progressive:49 Undetermined:1

Fate tests have been updated.

(In testing, it seems this filter will also need a subsequent patch for single frame input)

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