revised video renderer's invalidate cache to use ranges rather than discrete

frames

Previously, when the video renderer received a dirty cache signal, it would
proceed to extract all frames from the range and queue them. However, this could
be extremely slow for long ranges since it had to iterate through the entire
range and calculate the individual frames it contained. Now, we use the same
range combining system as audio and automatically calculate the next frame
within the range only when necessary. Essentially the same work, but split up
over time and done only when needed leading to no discernible UI pause when
invalidating cache.

app/render/backend/renderbackend.h

@@ -30,7 +30,7 @@ public:
   ViewerOutput* viewer_node() const;
 
 public slots:
-  virtual void InvalidateCache(const rational &start_range, const rational &end_range);
+  void InvalidateCache(const rational &start_range, const rational &end_range);
 
   bool Compile();
 
@@ -52,6 +52,8 @@ protected:
 
   virtual bool CanRender();
 
+  virtual TimeRange PopNextFrameFromQueue();
+
   rational GetSequenceLength();
 
   const QVector<QThread*>& threads();
@@ -98,6 +100,9 @@ protected:
 
   QHash<TimeRange, qint64> render_job_info_;
 
+protected slots:
+  void QueueRecompile();
+
 private:
   bool AllProcessorsAreAvailable() const;
 
@@ -136,9 +141,6 @@ private:
 
   QVector<bool> processor_busy_state_;
 
-private slots:
-  void QueueRecompile();
-
 };
 
 #endif // RENDERBACKEND_H