SEI: enable generation of recovery point SEI message for keyframes

SEI recovery points are inserted for every keyframe which tells the decoder an
identifier of the recovery point from which perfectly valid pictures can be
displayed no matter what the starting point of decoding is. The SEI specifies
recovery_poc_cnt that counts the number of frames after which perfect video can
be displayed to the user. Pictures encoded after the CRA but precede it in
display order(leading) are ignored by the decoder and pictures following CRA in
POC order(trailing) do not reference pictures prior to the recent CRA and are
guaranteed to be displayable. Hence recovery_poc_cnt is zero.