code: protected long getEstimatedCacheMemoryUsage ()
      {
          long size = 0;
          Iterator<CompositedMultiFrameImage> iter = _frameCache.values().iterator();
          while (iter.hasNext()) {
              size += iter.next().getEstimatedMemoryUsage();
          }
          return size;
      }
docs: Returns the estimated memory usage in bytes for all images
  currently cached by the cached action frames.