message: very coarse support for cancellation in DFB
diff: @@ -670,8 +670,7 @@ namespace ospray {
  globalTilesCompletedThisFrame += msg->numCompleted;
  const float progress = globalTilesCompletedThisFrame / (float)getTotalTiles();
  if (!api::currentDevice().reportProgress(progress)) {
  - // WILL TODO
  - //sendCancelRenderingMessage();
  + sendCancelRenderingMessage();
  }
  }
  @@ -926,6 +925,14 @@ namespace ospray {
  void DFB::sendCancelRenderingMessage()
  {
  + PING;
  + // WILL: Because we don't have a threaded MPI and the master
  + // in replicated rendering will just be waiting in the bcast when
  + // we call cancel, the message would never actually be sent out this
  + // frame. It would actually be delayed and sent the following frame.
  + // Instead, the best we can do without a threaded MPI at this point
  + // is to just cancel the final gathering stage.
  + cancelRendering = true;
  /*
  std::cout << "SENDING CANCEL RENDERING MSG\n";
  PING;
  