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/task/export/export.cpp

@@ -108,7 +108,7 @@ bool ExportTask::Run()
 
   if (params_.audio_enabled()) {
     // Write audio data now
-    encoder_->WriteAudio(audio_params(), audio_data_.GetPCMFilename());
+    encoder_->WriteAudio(audio_params(), audio_data_.CreatePlaybackDevice(encoder_));
   }
 
   encoder_->Close();