Fix NVENC Falling Back To VBR In CBR Mode

Util/FFmpeg/FFmpegNativeEncoder.cs

@@ -1080,6 +1080,14 @@ namespace ADOFAIRenderer.Util.FFmpeg
                         {
                             TrySetOption("x265-params", "hrd=1:strict-cbr=1");
                         }
+                        // NVENC: without "rc", nvenc_setup_rate_control selects CONSTQP when "qp" is set and VBR otherwise;
+                        //   bit_rate and rc_max_rate being equal does not select CBR. "rc=cbr" selects NV_ENC_PARAMS_RC_CBR.
+                        //   Reference: https://github.com/FFmpeg/FFmpeg/blob/release/8.0/libavcodec/nvenc.c (nvenc_setup_rate_control),
+                        //   https://github.com/FFmpeg/FFmpeg/blob/release/8.0/libavcodec/nvenc_h264.c ("rc" option, "cbr" constant)
+                        else if (encoder.Kind == EncoderKind.NVENC)
+                        {
+                            TrySetOption("rc", "cbr");
+                        }
                     }
                     break;