message: fix deadlock problem --> slave never called DFB::endFrame()
  [ci skip]
diff: @@ -76,7 +76,6 @@ namespace ospray {
  if ((ALLTASKS % worker.size) > worker.rank)
  NTASKS++;
  - //serial_for(NTASKS, [&](int taskIndex){
  parallel_for(NTASKS, [&](int taskIndex){
  const size_t tileID = taskIndex * worker.size + worker.rank;
  const size_t numTiles_x = fb->getNumTiles().x;
  @@ -97,7 +96,6 @@ namespace ospray {
  Tile __aligned(64) tile(tileId, fb->size, accumID);
  #endif
  - //serial_for(numJobs(renderer->spp, accumID), [&](int tid){
  parallel_for(numJobs(renderer->spp, accumID), [&](int tid){
  renderer->renderTile(perFrameData, tile, tid);
  });
  @@ -108,7 +106,8 @@ namespace ospray {
  dfb->waitUntilFinished();
  renderer->endFrame(perFrameData,channelFlags);
  - return inf; // irrelevant on slave
  + return dfb->endFrame(0.f); // irrelevant return value on slave, still
  + // call to stop maml layer
  }
  std::string Slave::toString() const
  