renderer: switched audio rendering over to planar buffers

While planar audio cannot be used for playback, it's much easier for processing.
This should lead to cleaner and easier to write code in the future.

app/codec/decoder.h

@@ -26,6 +26,7 @@
 #include <stdint.h>
 
 #include "codec/frame.h"
+#include "codec/samplebuffer.h"
 #include "codec/waveoutput.h"
 #include "common/constructors.h"
 #include "common/rational.h"
@@ -168,7 +169,7 @@ public:
    * A FramePtr of valid data at this timecode of the requested length or nullptr if there was nothing to retrieve at
    * the provided timecode or the media could not be opened.
    */
-  virtual FramePtr RetrieveAudio(const rational& timecode, const rational& length, const AudioRenderingParams& params);
+  virtual SampleBufferPtr RetrieveAudio(const rational& timecode, const rational& length, const AudioRenderingParams& params);
 
   virtual bool SupportsVideo();
   virtual bool SupportsAudio();