avio: Introduce avio_read_to_bprint(avioctx, bp, max_size)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavd/lavfi: work with non-mappable files for graph_file option

Example of non-mappable file is /dev/stdin. Previously passing it as
graph_file value returned error.

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