feat(498): QSV pipeline decodes via VA-API to software when preferring native decoder

ErsatzTV.FFmpeg/Decoder/DecoderVaapiToSoftware.cs

@@ -0,0 +1,19 @@
+using ErsatzTV.FFmpeg.Format;
+
+namespace ErsatzTV.FFmpeg.Decoder;
+
+// VA-API-accelerated decode that downloads frames to system memory (no
+// -hwaccel_output_format). Pairs with `-hwaccel vaapi` from
+// QsvHardwareAccelerationOption on the "prefer native decoder" QSV path: the
+// error-tolerant VA-API decoder feeds software frames into the QSV builder's
+// format=nv12,hwupload,vpp_qsv branch, which re-uploads for the QSV encoder.
+public class DecoderVaapiToSoftware : DecoderBase
+{
+    protected override FrameDataLocation OutputFrameDataLocation => FrameDataLocation.Software;
+
+    public override string Name => "implicit_vaapi";
+
+    // no -c:v (implicit decoder; `-hwaccel vaapi` drives VA-API) and no
+    // -hwaccel_output_format (frames download to software)
+    public override string[] InputOptions(InputFile inputFile) => [];
+}