Per FFmpeg commit 337f777f378c [1], FFmpeg removed nvenc_h264_encoder
and nvenc_hevc_encoder after deprecation in FFmpeg commit 888a5c794778
[2]. The names to be used are ff_h264_nvenc_encoder and
ff_hevc_nvenc_encoder. So we must allow alternative search of codec as
h264_nvenc or nvenc_h264 in obs-ffmpeg.c.
[1]: 337f777f37
[2]: 888a5c7947