This function was already getting very long and complex,
and we'll soon be moving it into two more for loops.

This also fixes a bug where output packets from the
encoder thread (with --sout-transcode-threads larger than zero)
could get lost, since we could extract the output blocks twice
and thus overwrite the output parameter.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>