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/src/api/kernels_core.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 Intel Corporation
+// Copyright (C) 2018-2020 Intel Corporation
 
 
 #include "precomp.hpp"
@@ -234,6 +234,11 @@ GScalar sum(const GMat& src)
     return core::GSum::on(src);
 }
 
+GOpaque<int> countNonZero(const GMat& src)
+{
+    return core::GCountNonZero::on(src);
+}
+
 GMat addWeighted(const GMat& src1, double alpha, const GMat& src2, double beta, double gamma, int dtype)
 {
     return core::GAddW::on(src1, alpha, src2, beta, gamma, dtype);