message: Fixes for newer TBB
diff: #include <tbb/blocked_range2d.h>
  #include <tbb/parallel_for.h>
  +#if 1 // TODO: find out when that API changed
  +#include <tbb/global_control.h>
  +#else
  #include <tbb/task_scheduler_init.h>
  +#endif
  #include "basic_sched.h"
  #include "range.h"
  @@ -19,13 +23,21 @@ namespace visionaray
  struct tbb_sched_backend
  {
  explicit tbb_sched_backend(unsigned num_threads)
  +#if 1 // TODO: find out when that API changed
  + : tbb_gc_(new tbb::global_control(tbb::global_control::max_allowed_parallelism, num_threads))
  +#else
  : init_(num_threads)
  +#endif
  {
  }
  void reset(unsigned num_threads)
  {
  +#if 1 // TODO: find out when that API changed
  + tbb_gc_.reset(new tbb::global_control(tbb::global_control::max_allowed_parallelism, num_threads));
  +#else
  init_.initialize(num_threads);
  +#endif
  }
  template <typename Func>
  @@ -59,7 +71,11 @@ struct tbb_sched_backend
  });
  }
  +#if 1 // TODO: find out when that API changed
  + std::unique_ptr<tbb::global_control> tbb_gc_;
  +#else
  tbb::task_scheduler_init init_;
  +#endif
  };
  template <typename R>
  