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/test/gapi_frame_tests.cpp

@@ -17,15 +17,13 @@ G_API_OP(GBlurFrame, <GMat(GFrame)>, "test.blur_frame") {
     }
 };
 
-GAPI_OCV_KERNEL(OCVBlurFrame, GBlurFrame)
-{
+GAPI_OCV_KERNEL(OCVBlurFrame, GBlurFrame) {
     static void run(const cv::Mat& in, cv::Mat& out) {
         cv::blur(in, out, cv::Size{3,3});
     }
 };
 
-struct GFrameTest : public ::testing::Test
-{
+struct GFrameTest : public ::testing::Test {
     cv::Size sz{32,32};
     cv::Mat in_mat;
     cv::Mat out_mat;
@@ -34,20 +32,17 @@ struct GFrameTest : public ::testing::Test
     GFrameTest()
         : in_mat(cv::Mat(sz, CV_8UC1))
         , out_mat(cv::Mat::zeros(sz, CV_8UC1))
-        , out_mat_ocv(cv::Mat::zeros(sz, CV_8UC1))
-    {
+        , out_mat_ocv(cv::Mat::zeros(sz, CV_8UC1)) {
         cv::randn(in_mat, cv::Scalar::all(127.0f), cv::Scalar::all(40.f));
         cv::blur(in_mat, out_mat_ocv, cv::Size{3,3});
     }
 
-    void check()
-    {
+    void check() {
         EXPECT_EQ(0, cvtest::norm(out_mat, out_mat_ocv, NORM_INF));
     }
 };
 
-TEST_F(GFrameTest, Input)
-{
+TEST_F(GFrameTest, Input) {
     cv::GFrame in;
     auto out = GBlurFrame::on(in);
     cv::GComputation c(cv::GIn(in), cv::GOut(out));