avcodec/mjpegdec: Improve intel jpeg flip heuristic

Fixes Ticket3698

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/dsputil: move some mmx init code inside dsputil_init_mmx()

This reduces differences with the fork

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Allow >2G for skip_initial_bytes.