Fix multithreaded MPEG-4 decoding.

Regression since c10d498bfd246a40cc7830838b74e18a79418839.
Unfortunately ff_thread_get_format can only be called from
a separate decode thread, running it during init will fail.
Fixes for that are welcome, for now just revert back to
calling avctx->get_format directly, which is correct
but having to decide on a case-by-case basis which approach
to use is a bit messy.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
mxfdec: fix return variable type for avio_seek

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>