fix hevc_nvenc encoder on sm < 75 (#1187)

ErsatzTV.FFmpeg/Capabilities/NvidiaHardwareCapabilities.cs

@@ -95,6 +95,8 @@ public class NvidiaHardwareCapabilities : IHardwareCapabilities
         return isHardware ? FFmpegCapability.Hardware : FFmpegCapability.Software;
     }
 
+    public bool HevcBFrames => _architecture >= 75;
+
     private FFmpegCapability CheckHardwareCodec(string codec, Func<string, bool> check)
     {
         if (check(codec))

ErsatzTV.FFmpeg/Encoder/Nvenc/EncoderHevcNvenc.cs

@@ -1,13 +1,27 @@
-using ErsatzTV.FFmpeg.Format;
+using ErsatzTV.FFmpeg.Capabilities;
+using ErsatzTV.FFmpeg.Format;
 
 namespace ErsatzTV.FFmpeg.Encoder.Nvenc;
 
 public class EncoderHevcNvenc : EncoderBase
 {
+    private readonly bool _bFrames;
+    
+    public EncoderHevcNvenc(IHardwareCapabilities hardwareCapabilities)
+    {
+        if (hardwareCapabilities is NvidiaHardwareCapabilities nvidia)
+        {
+            _bFrames = nvidia.HevcBFrames;
+        }
+    }
+
     public override string Name => "hevc_nvenc";
     public override StreamKind Kind => StreamKind.Video;
     public override FrameState NextState(FrameState currentState) => currentState with
     {
         VideoFormat = VideoFormat.Hevc
     };
+
+    public override IList<string> OutputOptions =>
+        new[] { "-c:v", "hevc_nvenc", "-b_ref_mode", _bFrames ? "1" : "0" };
 }

ErsatzTV.FFmpeg/Pipeline/NvidiaPipelineBuilder.cs

@@ -223,7 +223,7 @@ public class NvidiaPipelineBuilder : SoftwarePipelineBuilder
             Option<IEncoder> maybeEncoder =
                 (ffmpegState.EncoderHardwareAccelerationMode, desiredState.VideoFormat) switch
                 {
-                    (HardwareAccelerationMode.Nvenc, VideoFormat.Hevc) => new EncoderHevcNvenc(),
+                    (HardwareAccelerationMode.Nvenc, VideoFormat.Hevc) => new EncoderHevcNvenc(_hardwareCapabilities),
                     (HardwareAccelerationMode.Nvenc, VideoFormat.H264) => new EncoderH264Nvenc(),
 
                     (_, _) => GetSoftwareEncoder(currentState, desiredState)