avcodec/dvbsubdec: restructure version check

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dvbsubdec: add AVClass to context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dvbsubdec: Split save_subtitle_set() out

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