* core, stitching: revise syntax to support Visual C++ 2013

* stitching: revise syntax again to support Visual C++ 2013 and other compilers

* stitching: minor update to clarify changes

* Enabled -Wnarrowing warning

* Fixed type narrowing issues

* Cast python constants

* Use long long for python constants

* Use int for python constants with fallback to long

* Update cv2.cpp

- Use shfl_down, instead of __shfl_down, on warp scan
- Remove race conditions

New computeECC function, and updated findTransformECC function to make gaussian filtering optional (#13837)

* fix for https://github.com/opencv/opencv/issues/12432 with doc and tests

* Added doc string for new parameter.

* Fixes suggested by Alalek for getting around ABI incompatibility.

* Update to docstring, to remove parameter that isn't relevant.

* More updates based on Alalek's usggestions.