add watermark support to experimental transcoder logic (#654)

* wip

* fix songs again

* don't use lists of video and audio input files

* test concat command output

* move concat pipeline

* start to use ffmpeg state

* add ffmpeg state and audio state

* audio state is required

* attach input options directly to input files

* move filters to input files

* add watermark support

ErsatzTV.FFmpeg/Encoder/EncoderBase.cs

@@ -9,7 +9,7 @@ public abstract class EncoderBase : IEncoder
     public IList<string> InputOptions(InputFile inputFile) => Array.Empty<string>();
     public IList<string> FilterOptions => Array.Empty<string>();
     public virtual IList<string> OutputOptions => new List<string> { Kind == StreamKind.Video ? "-c:v" : "-c:a", Name };
-    public abstract FrameState NextState(FrameState currentState);
+    public virtual FrameState NextState(FrameState currentState) => currentState;
 
     public abstract string Name { get; }
     public abstract StreamKind Kind { get; }