Semantic for the function ff_null_start_frame() was changed in
07bad27810cd, and it has now the same behavior of
ff_null_start_frame_keep_ref(), thus it makes no sense to keep both of
them.
Also add internal function ff_null_start_frame_keep_ref().
Fix crash when a following filter (e.g. settb) will unref the reference
passed by start_frame(), and then the reference is accessed in
end_frame() through inlink->cur_buf.
Also add bbox.h and bbox.c files, based on the remove-logo filter by
Robert Edele. These files are useful for sharing code with the pending
removelogo port.