Merge pull request #18391 from dmatveev:dm/gframe_00_new_type

* G-API: Make GFrame a new (distinct) G-type, not an alias to GMat

- The underlying host type is still cv::Mat, a new cv::MediaFrame
  type is to be added as a separate PR

* Fix warnings and review comments

- Somewhow there was a switch() without a default: clause in Fluid

modules/gapi/src/api/gframe.cpp

@@ -0,0 +1,41 @@
+// 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/gframe.hpp>
+
+#include "api/gorigin.hpp"
+
+// cv::GFrame public implementation //////////////////////////////////////////////
+cv::GFrame::GFrame()
+    : m_priv(new GOrigin(GShape::GMAT, GNode::Param())) {
+    // N.B.: The shape here is still GMAT as currently cv::Mat is used
+    // as an underlying host type. Will be changed to GFRAME once
+    // GExecutor & GStreamingExecutor & selected backends will be extended
+    // to support cv::MediaFrame.
+}
+
+cv::GFrame::GFrame(const GNode &n, std::size_t out)
+    : m_priv(new GOrigin(GShape::GMAT, n, out)) {
+    // N.B.: GMAT is here for the same reason as above ^
+}
+
+cv::GOrigin& cv::GFrame::priv() {
+    return *m_priv;
+}
+
+const cv::GOrigin& cv::GFrame::priv() const {
+    return *m_priv;
+}
+
+namespace cv {
+std::ostream& operator<<(std::ostream& os, const cv::GFrameDesc &) {
+    return os;
+}
+
+} // namespace cv

modules/gapi/src/backends/fluid/gfluidbackend.cpp

@@ -1249,6 +1249,7 @@ void cv::gimpl::GFluidExecutable::bindInArg(const cv::gimpl::RcDesc &rc, const G
     case GShape::GSCALAR: m_res.slot<cv::Scalar>()[rc.id] = util::get<cv::Scalar>(arg); break;
     case GShape::GARRAY:  m_res.slot<cv::detail::VectorRef>()[rc.id] = util::get<cv::detail::VectorRef>(arg); break;
     case GShape::GOPAQUE: m_res.slot<cv::detail::OpaqueRef>()[rc.id] = util::get<cv::detail::OpaqueRef>(arg); break;
+    default: util::throw_error(std::logic_error("Unsupported input GShape type"));
     }
 }