avfilter/vf_cropdetect: support 9-16bit planar formats

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_cropdetect: fix ; typos

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_cropdetect: extend limit to cover 16bit pixel formats

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