avcodec/mpegvideo: Fix block height for lowres 3 interlaced blocks

Fixes green trash
Fixes part of Ticket2535

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpegvideo: Fix edge emu with lowres

Fixes a few green artifacts at the top
Fixes rest of Ticket 2535

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>