message: Fix crash w/ old data-parallel vol. renderer and variance
  We'd call begin frame before startNewFrame so would call tileError sync
  while MAML was running, and crash.
diff: @@ -261,10 +261,9 @@ namespace ospray {
  }
  dfb->setFrameMode(DistributedFrameBuffer::ALPHA_BLEND);
  -
  + dfb->startNewFrame(errorThreshold);
  Renderer::beginFrame(fb);
  - dfb->startNewFrame(errorThreshold);
  if (ddVolumeVec.size() > 1) {
  /* note: our assumption below is that all blocks together are
  