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

diff --cc modules/core/include/opencv2/core/mat.hpp
Simple merge

diff --cc modules/core/include/opencv2/core/operations.hpp
Simple merge

diff --cc modules/features2d/src/draw.cpp
Simple merge

diff --cc modules/highgui/src/window_w32.cpp
Simple merge

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

diff --cc modules/imgproc/test/test_filter.cpp
Simple merge

diff --cc modules/stitching/src/matchers.cpp
Simple merge