lavfi/lut: Add slice threading support

Used the command for 1080p h264 clip as follow:

a). ffmpeg -i input -vf lutyuv="u=128:v=128" -f null /dev/null
b). ffmpeg -i input -vf lutrgb="g=0:b=0" -f null /dev/null

after enabled the slice threading, the fps change from:

a). 144fps to 258fps (lutyuv)
b). 94fps  to 153fps (lutrgb)

in Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>