vda_h264_dec: backup context before overriding

Some of context fields which must be overrided by this wrapper decoder
may be set and used by user, so we have to save and restore them before
and after invoking the inner decoder to secure user.
Merge branch 'master' of https://github.com/upsuper/ffmpeg-vdadec

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