Merge pull request #18415 from dmatveev:dm/gframe_01_new_host_type

* G-API: Introduce cv::MediaFrame, a host type for cv::GFrame

* G-API: RMat -- address review comments

modules/gapi/src/api/media.cpp

@@ -0,0 +1,42 @@
+// 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/media.hpp>
+
+struct cv::MediaFrame::Priv {
+    std::unique_ptr<IAdapter> adapter;
+};
+
+cv::MediaFrame::MediaFrame() {
+}
+
+cv::MediaFrame::MediaFrame(AdapterPtr &&ptr)
+    : m(new Priv{std::move(ptr)}) {
+}
+
+cv::GFrameDesc cv::MediaFrame::desc() const {
+    return m->adapter->meta();
+}
+
+cv::MediaFrame::View cv::MediaFrame::access(Access code) {
+    return m->adapter->access(code);
+}
+
+cv::MediaFrame::View::View(Ptrs&& ptrs, Strides&& strs, Callback &&cb)
+    : ptr   (std::move(ptrs))
+    , stride(std::move(strs))
+    , m_cb  (std::move(cb)) {
+}
+
+cv::MediaFrame::View::~View() {
+    if (m_cb) {
+        m_cb();
+    }
+}
+
+cv::MediaFrame::IAdapter::~IAdapter() {
+}