message: Move semantics for blocking_queue::push_back()
diff: #include <deque>
  #include <memory>
  #include <mutex>
  +#include <utility>
  #include <visionaray/detail/semaphore.h>
  @@ -36,6 +37,17 @@ public:
  non_empty_.notify();
  }
  + void push_back(T&& value)
  + {
  + {
  + std::unique_lock<std::mutex> l(mutex_);
  +
  + queue_.push_back(std::move(value));
  + }
  +
  + non_empty_.notify();
  + }
  +
  T pop_front()
  {
  T next;
  