decoder: simplify based on vout_GetPicture() change
decoder: simplify based on vout_GetPicture() change

Also fixes lot of spurious decoder errors when stopping due to
excessive flush checks.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc