avcodec/h263dec: Check mmb parameters

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg4video: also ignore DC over/underflows when error detection is set to ignore errors

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg4video: implement non relative mmb blocks which dont depend on previous blocks

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h263dec: fix buffer padding