cache: reimplemented auto-caching

Implemented a smarter auto-cache that's context sensitive. Caching will
automatically pause when playback begins or values are being changed and resume
when inactive.

app/task/render/render.h

@@ -57,6 +57,8 @@ protected:
     return audio_params_;
   }
 
+  void SetAnchorPoint(const rational& r);
+
 private:
   ViewerOutput* viewer_;
 
@@ -64,6 +66,8 @@ private:
 
   AudioRenderingParams audio_params_;
 
+  rational anchor_point_;
+
 };
 
 OLIVE_NAMESPACE_EXIT