audio: rewrote audio cache to write to contiguous segments rather than one long file

Fixes issues with PCM file placement, optimizes some audio-based timeline
and cache operations, just generally a better approach.

app/codec/encoder.h

@@ -116,7 +116,9 @@ public:
 
   virtual bool WriteFrame(OLIVE_NAMESPACE::FramePtr frame, OLIVE_NAMESPACE::rational time) = 0;
   virtual void WriteAudio(OLIVE_NAMESPACE::AudioParams pcm_info,
-                          const QString& pcm_filename) = 0;
+                          QIODevice *file) = 0;
+  void WriteAudio(OLIVE_NAMESPACE::AudioParams pcm_info,
+                  const QString& pcm_filename);
 
   virtual void Close() = 0;