Merge pull request #18451 from OrestChura:oc/count_non_zero

[G-API]: countNonZero() Standard Kernel Implementation

* Add countNonZero() standard kernel
 - API and documentation provided
 - OCV backend supported
 - accuracy and performance tests provided
 - some refactoring of related documentation done

* Fix GOpaque functionality for OCL Backend
 - test for OCL Opaque usage providied

* countNonZero for GPU
 - OCL Backend implementation for countNonZero() added
 - tests provided

* Addressing comments

modules/gapi/test/cpu/gapi_core_tests_cpu.cpp

@@ -2,7 +2,7 @@
 // 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) 2018-2019 Intel Corporation
+// Copyright (C) 2018-2020 Intel Corporation
 
 
 #include "../test_precomp.hpp"
@@ -202,6 +202,15 @@ INSTANTIATE_TEST_CASE_P(SumTestCPU, SumTest,
                                 Values(CORE_CPU),
                                 Values(AbsToleranceScalar(1e-5).to_compare_obj())));
 
+INSTANTIATE_TEST_CASE_P(CountNonZeroTestCPU, CountNonZeroTest,
+                        Combine(Values( CV_8UC1, CV_16UC1, CV_16SC1, CV_32FC1 ),
+                                Values(cv::Size(1280, 720),
+                                       cv::Size(640, 480),
+                                       cv::Size(128, 128)),
+                                Values(-1),
+                                Values(CORE_CPU),
+                                Values(AbsToleranceScalar(1e-5).to_compare_obj())));
+
 INSTANTIATE_TEST_CASE_P(AbsDiffTestCPU, AbsDiffTest,
                         Combine(Values( CV_8UC1, CV_8UC3, CV_16UC1, CV_16SC1, CV_32FC1 ),
                                 Values(cv::Size(1280, 720),