obu: check that the frame referenced by existing_frame_idx is showable

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, that the value of showable_frame for the previous frame was equal to 1."

"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."