avcodec/nvenc: Add 'nvenc_h264' as an alternative name for 'nvenc'

This allows us to offer the same codec name that libav uses. We don't have
a special way to do aliases, so it's all a bit more verbose than you'd want
but such is life.

Signed-off-by: Philip Langdale <philipl@overt.org>

libavcodec/allcodecs.c

@@ -236,6 +236,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER(MXPEG,             mxpeg);
     REGISTER_DECODER(NUV,               nuv);
     REGISTER_ENCODER(NVENC,             nvenc);
+    REGISTER_ENCODER(NVENC_H264,        nvenc_h264);
     REGISTER_ENCODER(NVENC_HEVC,        nvenc_hevc);
     REGISTER_DECODER(PAF_VIDEO,         paf_video);
     REGISTER_ENCDEC (PAM,               pam);