lavfi/blend: use correct way to check number of planes

This fix crash with gray, as its marked as pseudopal,
and thus have extra plane.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
lavfi/blend: merge config_input_top into config_output

Signed-off-by: Paul B Mahol <onemda@gmail.com>
lavu/opencl: fix bug of opencl options

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