Blackframe video filter now sets metadata accordingly.

the libavfilter/vf_blackframe.c filter now not only logs detected
values, but also sets frame metadata. Currently, only `pblack` value is set
but `SET_META` macro has been introduced to ease development in the future.

Signed-off-by: Stepan Bujnak <stepan.bujnak@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>