gapi: Full calcOpticalFlowPyrLK implementation (2 overloads) and tests

- opencv_gapi module is linked with opencv_video module (optional dependency)
     - kernels added to a new cv::gapi::video namespace and a brand new files created to provide gapi_video environment
     - there are 2 different kernels as G-API should provide GMat AND GArray<GMat> implementation: cv::calcOptFlowPyrLK doesn't calculate pyramids if vector<Mat> is given so just the cast GMat -> GArray<GMat> wouldn't represent all the cv:: functionality
     - tests to check both kernels (based on cv::video tests for cv::calcOpticalFlowPyrLK())
     - tests for internal purposes added
     - vectors<T> comparison in tests implemented
     - new (and old too) common test structures refactored to avoid code copypasting
     - "modules/gapi/test/common/gapi_video_tests_common.hpp" created to share some code snippets between perf and acc tests and avoid code copypasting

modules/gapi/src/api/kernels_video.cpp

@@ -0,0 +1,45 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+//
+// Copyright (C) 2020 Intel Corporation
+
+
+#include "precomp.hpp"
+
+#include <opencv2/gapi/video.hpp>
+
+namespace cv { namespace gapi {
+using namespace video;
+
+
+GOptFlowLKOutput calcOpticalFlowPyrLK(const GMat                    &prevImg,
+                                      const GMat                    &nextImg,
+                                      const cv::GArray<cv::Point2f> &prevPts,
+                                      const cv::GArray<cv::Point2f> &predPts,
+                                      const Size                    &winSize,
+                                            int                      maxLevel,
+                                      const TermCriteria            &criteria,
+                                            int                      flags,
+                                            double                   minEigThresh)
+{
+    return GCalcOptFlowLK::on(prevImg, nextImg, prevPts, predPts, winSize, maxLevel,
+                              criteria, flags, minEigThresh);
+}
+
+GOptFlowLKOutput calcOpticalFlowPyrLK(const cv::GArray<cv::GMat>    &prevPyr,
+                                      const cv::GArray<cv::GMat>    &nextPyr,
+                                      const cv::GArray<cv::Point2f> &prevPts,
+                                      const cv::GArray<cv::Point2f> &predPts,
+                                      const Size                    &winSize,
+                                            int                      maxLevel,
+                                      const TermCriteria            &criteria,
+                                            int                      flags,
+                                            double                   minEigThresh)
+{
+    return GCalcOptFlowLKForPyr::on(prevPyr, nextPyr, prevPts, predPts, winSize, maxLevel,
+                                    criteria, flags, minEigThresh);
+}
+
+} //namespace gapi
+} //namespace cv