message: dnn(caffe): add DetectionOutputParameter.clip to .proto file
  allow to load opencv_face_detector with external unpatched protobuf
diff: @@ -181,6 +181,8 @@ message DetectionOutputParameter {
  optional float confidence_threshold = 9;
  // If prior boxes are normalized to [0, 1] or not.
  optional bool normalized_bbox = 10 [default = true];
  + // OpenCV custom parameter
  + optional bool clip = 1000 [default = false];
  }
  message Datum {
  