There is currently no way for API users to know that a buffersrc is no longer
accepting input, except by trying to feed it a frame and seeing what happens.
Of course, this is not possible if the user does not *have* a frame to feed,
but may still wish to know if the filter is still accepting input or not.
Since passing `frame == NULL` to `av_buffersrc_add_frame()` is already treated
as closing the input, we are left with no choice but to introduce a new
function for this.
We don't explicitly return the result of `ff_outlink_get_status()` to avoid
leaking internal status codes, and instead translate them all to AVERROR(EOF).
623669a02c