message: clear the fb variance when resetting tfcn as well
diff: @@ -366,7 +366,7 @@ int main(int argc, char **argv) {
  volume.tfcn.set("opacities", alphaData);
  volume.tfcn.commit();
  - fb.clear(OSP_FB_COLOR | OSP_FB_ACCUM);
  + fb.clear(OSP_FB_COLOR | OSP_FB_ACCUM | OSP_FB_VARIANCE);
  app.tfcnChanged = false;
  }
  }
  