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/codec/oiio/oiiodecoder.h

@@ -40,7 +40,6 @@ public:
   virtual bool Probe(Footage *f, const QAtomicInt* cancelled) override;
 
   virtual bool Open() override;
-  virtual RetrieveState GetRetrieveState(const rational &time) override;
   virtual FramePtr RetrieveVideo(const rational &timecode, const int& divider) override;
   virtual void Close() override;