framehashcache: allow access to frames being saved

Minor optimization and crash fix. If a frame is currently being
saved, it's still technically available in memory and can be
returned faster than loading from EXR. This also prevents
OpenEXR from trying to load a frame that we're midway through
saving.

app/render/framehashcache.h

@@ -92,6 +92,9 @@ private:
 
   rational timebase_;
 
+  static QMutex currently_saving_frames_mutex_;
+  static QMap<QByteArray, FramePtr> currently_saving_frames_;
+
 private slots:
   void HashDeleted(const QString &s, const QByteArray& hash);