* vget_lane_** accepts constant value, not variable

  * add 8 elements version of reduce operation
  * add tests for new universal intrinsic

checks whether the window exists and is visible. On QT closing a window
merley hides it, so the common hack for checking whether a window exists

exists = cv2.getWindowProperty(.., 0) >= 0

does not work.

Allow to use custom implementations of CV_XADD (passed via -D of CXXFLAGS)

Intel supports __atomic_fetch_add, so it isn't necessary to use
a specific version otherwise.

The optimization is for CV_8UC1 format and 3x3 box filter,
it is 15%~87% faster than current ocl kernel with below perf test

./modules/ts/misc/run.py -t imgproc --gtest_filter=OCL_BlurFixture*

Also add test cases for this ocl kernel.
Signed-off-by: Li Peng <peng.li@intel.com>

A bug in ICC improperly identified the first parameter as "void*"
rather than the proper "volatile long*".  This is scheduled to be
fixed in ICC in a future release.

This patch casts only to a "long*" to preserve backwards compatibility
with the ICC 16 and ICC 17 releases.