In output_packet(), move the pre_process_video_frame() call inside the
if (ist->decoding_needed) { } block. This way
pre_process_video_frame() is not called when stream-copy has been
selected.

Also simplify.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>