avcodec: Implement mpeg1 nvdec hwaccel

Once I remembered that there's a separate decoder type for mpeg1,
even though params struct is shared with mpeg2, everything worked.

libavcodec/allcodecs.c

@@ -87,6 +87,7 @@ static void register_all(void)
     REGISTER_HWACCEL(HEVC_VIDEOTOOLBOX, hevc_videotoolbox);
     REGISTER_HWACCEL(MJPEG_CUVID,       mjpeg_cuvid);
     REGISTER_HWACCEL(MPEG1_CUVID,       mpeg1_cuvid);
+    REGISTER_HWACCEL(MPEG1_NVDEC,       mpeg1_nvdec);
     REGISTER_HWACCEL(MPEG1_XVMC,        mpeg1_xvmc);
     REGISTER_HWACCEL(MPEG1_VDPAU,       mpeg1_vdpau);
     REGISTER_HWACCEL(MPEG1_VIDEOTOOLBOX, mpeg1_videotoolbox);