lavfi/movie: flush decoder frames on EOF

movie currently forwards EOF (or begins looping) immediately upon
reaching the last packet in the input stream, which can leave frames in
the decoder. This change first tries to read any remaining packets from
the decoder before forwarding EOF.

Signed-off-by: Steven Robertson <steven@strobe.cc>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
