began audio conform code in ffmpegdecoder

For accuracy, whenever we need to perform an audio resample, we need to do it
in advance. This is in a process called "conforming" and this commit introduces
the framework by which the decoder can automatically conform an audio stream
to arbitrary parameters for accurate rendering.

app/decoder/decoder.cpp

@@ -134,3 +134,10 @@ DecoderPtr Decoder::CreateFromID(const QString &id)
 
   return nullptr;
 }
+
+void Decoder::Conform(const AudioRenderingParams &params)
+{
+  Q_UNUSED(params)
+  qCritical() << "Conform called on an audio decoder that does not have a handler for it:" << id();
+  abort();
+}