avfilter/pthread: use slice threading from avutil

Benchmark (with 2 cpus):
./ffmpeg -f rawvideo -s 1280x720 -t 1000 -i /dev/zero \
    -filter_threads $threads -vf transpose=clock -f null null
threads=2:
  old: 31.129s 31.446s 31.574s
  new: 29.602s 29.636s 29.656s
threads=3 (nb_threads = nb_cpus + 1 is bad choice at this situation):
  old: 40.132s 40.279s 40.279s
  new: 39.308s 39.570s 39.693s
threads=4:
  old: 31.306s 31.366s 31.654s
  new: 30.231s 30.360s 30.451s

Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>