After inspecting the source code of x265, mpv and ffmpeg, I've found that
ffmpeg mistakenly regards EVC_NAL_BLA_N_LP and HEVC_NAL_IDR_N_LP as non-
reference frames, which are acutally reference frames according to the
specification in x265, and drops them.

This patch should address the problem. I have tested it with mpv.
Signed-off-by: Mark Wu <wfwf1997@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>