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

diff --cc modules/calib3d/src/circlesgrid.cpp
Simple merge

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

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

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

diff --cc modules/imgproc/src/precomp.hpp
Simple merge

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

diff --cc modules/ml/test/test_mltests2.cpp
Simple merge