message: Fix mutex lifetime in waitForCommands.
  This fixes the regression that I recently introduced in the
  single-threaded configuration commit. Many thanks to Mathias and Romain
  for catching this.
diff: @@ -102,12 +102,13 @@ void CommandBufferQueue::flush() noexcept {
  }
  std::vector<CommandBufferQueue::Slice> CommandBufferQueue::waitForCommands() const {
  - if (UTILS_HAS_THREADING) {
  + if (!UTILS_HAS_THREADING) {
  + return std::move(mCommandBuffersToExecute);
  + }
  std::unique_lock<utils::Mutex> lock(mLock);
  while (mCommandBuffersToExecute.empty() && !mExitRequested) {
  mCondition.wait(lock);
  }
  - }
  return std::move(mCommandBuffersToExecute);
  }
  