Merge remote-tracking branch 'upstream/3.4' into merge-3.4

diff --cc modules/core/include/opencv2/core/cvdef.h
Simple merge

diff --cc modules/core/src/arithm.cpp
Simple merge

diff --cc modules/core/src/persistence_cpp.cpp
Simple merge

diff --cc modules/core/test/test_mat.cpp
Simple merge

diff --cc modules/dnn/include/opencv2/dnn/dnn.hpp
Simple merge

diff --cc modules/dnn/src/dnn.cpp
Simple merge

diff --cc modules/dnn/src/layers/detection_output_layer.cpp
Simple merge

diff --cc modules/dnn/src/layers/eltwise_layer.cpp
Simple merge

diff --cc modules/dnn/src/layers/pooling_layer.cpp
Simple merge

diff --cc modules/dnn/src/layers/recurrent_layers.cpp
Simple merge

diff --cc modules/highgui/src/window.cpp
Simple merge

diff --cc modules/imgcodecs/src/loadsave.cpp
Simple merge

diff --cc modules/imgproc/src/drawing.cpp
Simple merge

diff --cc modules/imgproc/src/featureselect.cpp
Simple merge

diff --cc modules/imgproc/src/pyramids.cpp
Simple merge

diff --cc modules/objdetect/src/cascadedetect.cpp
Simple merge

diff --cc modules/video/src/compat_video.cpp
Simple merge

diff --cc modules/videoio/src/cap_openni2.cpp
Simple merge

diff --cc modules/videoio/src/cap_v4l.cpp
Simple merge

diff --cc modules/videoio/src/videoio_registry.cpp
Simple merge