separated decoder retrieve functions for video and audio

It became increasingly clear that video and audio retrieval require different
parameters so it started to make sense to separate these functions out.

app/decoder/decoder.cpp

@@ -55,6 +55,26 @@ void Decoder::set_stream(StreamPtr fs)
   stream_ = fs;
 }
 
+FramePtr Decoder::RetrieveVideo(const rational &/*timecode*/)
+{
+  return nullptr;
+}
+
+FramePtr Decoder::RetrieveAudio(const rational &/*timecode*/, const rational &/*length*/, const AudioRenderingParams &/*params*/)
+{
+  return nullptr;
+}
+
+bool Decoder::SupportsVideo()
+{
+  return false;
+}
+
+bool Decoder::SupportsAudio()
+{
+  return false;
+}
+
 /*
  * DECODER STATIC PUBLIC MEMBERS
  */