obu: don't output invisible but showable key frames more than once

From section 6.8.2 in the AV1 spec:

"It is a requirement of bitstream conformance that when show_existing_frame is
used to show a previous frame with RefFrameType[ frame_to_show_map_idx ] equal
to KEY_FRAME, that the frame is output via the show_existing_frame mechanism at
most once."