avfilter/vf_cropdetect: Unroll 1byte per sample loop

12807 -> 6336 decicycles

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_cropdetect: Unroll 3 & 4 bytes per sample loop

19484 -> 10266 decicycles

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>