avcodec: Add field order information to AVCodecParserContext.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
mpegvideo_parser: Set field_order.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
vc1_parser: Set field_order.

Signed-off-by: Anton Khirnov <anton@khirnov.net>