renderer/decoder: simplified audio PCM transcode

Turned the two-step PCM transcode into one step and simplified/removed much of
the unnecessary infrastructure that supported it. This makes the code cleaner
and generally improves the code paths.

app/task/conform/conform.cpp

@@ -42,7 +42,7 @@ void ConformTask::Action()
 
     connect(decoder.get(), &Decoder::IndexProgress, this, &ConformTask::ProgressChanged);
 
-    decoder->Conform(params_, &IsCancelled());
+    decoder->ConformAudio(&IsCancelled(), params_);
 
     emit Succeeded();
   }