Currently, it needs to be initialized by the ER caller (which is
currently either a mpegvideo decoder or h264dec). However, since none of
those decoders use MECmpContext for anything except ER, it makes more
sense to handle it purely inside ER.
This is done to disentangle ER from mpegvideo. In order to use a
classic Picture, callers can use ff_mpeg_set_erpic() or use a custom function
to set the fields. Please note that buffers need to be allocated before
calling ff_er_frame_end().
