cache: began base implementation of "shifting" the cache

Should be a fairly important optimization to make.

app/node/output/viewer/viewer.cpp

@@ -87,6 +87,12 @@ QString ViewerOutput::Description() const
   return tr("Interface between a Viewer panel and the node system.");
 }
 
+void ViewerOutput::ShiftCache(const rational &from, const rational &to)
+{
+  video_frame_cache_.Shift(from, to);
+  audio_playback_cache_.Shift(from, to);
+}
+
 void ViewerOutput::InvalidateCache(const TimeRange &range, NodeInput *from, NodeInput *source)
 {
   emit GraphChangedFrom(source);

app/node/output/viewer/viewer.h

@@ -54,6 +54,8 @@ public:
   virtual QList<CategoryID> Category() const override;
   virtual QString Description() const override;
 
+  void ShiftCache(const rational& from, const rational& to);
+
   NodeInput* texture_input() const {
     return texture_input_;
   }