Fix frame rate probing for interlaced MKV files

MediaBrowser.MediaEncoding/Probing/ProbeResultNormalizer.cs

@@ -666,6 +666,16 @@ namespace MediaBrowser.MediaEncoding.Probing
                 stream.AverageFrameRate = GetFrameRate(streamInfo.AverageFrameRate);
                 stream.RealFrameRate = GetFrameRate(streamInfo.RFrameRate);
 
+                // Interlaced video streams in Matroska containers return the field rate instead of the frame rate
+                // as both the average and real frame rate, so we half the returned frame rates to get the correct values
+                //
+                // https://gitlab.com/mbunkus/mkvtoolnix/-/wikis/Wrong-frame-rate-displayed
+                if (stream.IsInterlaced && formatInfo.FormatName.Contains("matroska", StringComparison.OrdinalIgnoreCase))
+                {
+                    stream.AverageFrameRate /= 2;
+                    stream.RealFrameRate /= 2;
+                }
+
                 if (isAudio || string.Equals(stream.Codec, "gif", StringComparison.OrdinalIgnoreCase) ||
                     string.Equals(stream.Codec, "png", StringComparison.OrdinalIgnoreCase))
                 {