message: dnn: add runtime parameter OPENCV_DNN_BACKEND_DEFAULT
  to control DNN_BACKEND_DEFAULT enumeration value behavior
diff: @@ -66,6 +66,15 @@ static bool DNN_DISABLE_MEMORY_OPTIMIZATIONS = utils::getConfigurationParameterB
  static bool DNN_OPENCL_ALLOW_ALL_DEVICES = utils::getConfigurationParameterBool("OPENCV_DNN_OPENCL_ALLOW_ALL_DEVICES", false);
  #endif
  +static int PARAM_DNN_BACKEND_DEFAULT = (int)utils::getConfigurationParameterSizeT("OPENCV_DNN_BACKEND_DEFAULT",
  +#ifdef HAVE_INF_ENGINE
  + (size_t)DNN_BACKEND_INFERENCE_ENGINE
  +#else
  + (size_t)DNN_BACKEND_OPENCV
  +#endif
  +);
  +
  +
  using std::vector;
  using std::map;
  using std::make_pair;
  @@ -851,11 +860,8 @@ struct Net::Impl
  CV_TRACE_FUNCTION();
  if (preferableBackend == DNN_BACKEND_DEFAULT)
  -#ifdef HAVE_INF_ENGINE
  - preferableBackend = DNN_BACKEND_INFERENCE_ENGINE;
  -#else
  - preferableBackend = DNN_BACKEND_OPENCV;
  -#endif
  + preferableBackend = (Backend)PARAM_DNN_BACKEND_DEFAULT;
  +
  CV_Assert(preferableBackend != DNN_BACKEND_OPENCV ||
  preferableTarget == DNN_TARGET_CPU ||
  preferableTarget == DNN_TARGET_OPENCL ||
  