avfilter/vf_minterpolate: change the default threshold to get better scene change...

avfilter/vf_minterpolate: change the default threshold to get better scene change detect result

 ./ffmpeg -loglevel debug -i ../fate-suite/svq3/Vertical400kbit.sorenson3.mov -vf
 minterpolate=fps=60:mi_mode=blend -an -f null -
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 1600
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 4120
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 5780
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 6700
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 8140
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 9740
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 14060
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 15680
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 18480
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 20020
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 21740

 The results are consistent with tests/ref/fate/filter-metadata-scenedetect

 For the master, it'll detect more than 20 scene change for the same source.

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>