render: queue audio alongside video

Previously, audio was all queued first and would therefore all have to
finish before any video frames could start caching. No longer! Now they're
queued side by side so users won't have to wait for audio to finish before
their cached frames come in.

app/render/backend/renderbackend.h

@@ -87,7 +87,7 @@ public:
 
   void SetVideoDownloadMatrix(const QMatrix4x4& mat);
 
-  static QList<TimeRange> SplitRangeIntoChunks(const TimeRange& r);
+  static std::list<TimeRange> SplitRangeIntoChunks(const TimeRange& r);
 
 public slots:
   void NodeGraphChanged(NodeInput *source);