lavc: rework handling of refcounted_frames=0

Use only proper AVFrame API (no assigning of whole frames, since that
hardcodes sizeof(AVFrame) into lavc).

Make a copy of the side data, so the caller can use av_frame_unref/free
on non-refcounted frames, eliminating the need for
avcodec_get_frame_defaults()/avcodec_free_frame().