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/imgcodecs/include/opencv2/imgcodecs.hpp
Simple merge

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

diff --cc modules/js/CMakeLists.txt
Simple merge

diff --cc platforms/js/build_js.py
Simple merge