Only one codec using mjpegdec.c actually creates multiple
frames from a single packet, namely SMVJPEG. The other can
use the ordinary decode callback just fine. This e.g. has
the advantage of confining the special SP5X/AMV code to sp5xdec.c.

This reverts most of commit e9a2a877;
of course it is not a simple revert: Way too much has changed;
furthermore, outright reverting the sp5xdec.c changes would readd
a stack packet to sp5x_decode_frame() which is not desired.
In order to avoid this without modifying the given AVPacket,
a variant of ff_mjpeg_decode_frame() with explicit buf and size
parameters has been added.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>