ffv1dec: Avoid unnecessarily large stack usage and copies.

Ideally the compiler could figure this out on its own,
but it seems it can't.
An alternative that would avoid the messy explicit memcpy
would be to use a sub-struct for the parts that should
be preserved, which can then simply be assigned.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
avcodec/mjpegdec: fix pixfmtid 0x14111100

Fixes part of Ticket 2004
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>