Merge pull request #15888 from OrestChura:facebeautification_gapi_sample

Introducing the sample of Face Beautification algorithm implemented via Graph-API

* Introducing the sample of Face Beautification algorithm implemented via Graph-API
- 'gapi/samples/face_beautification.cpp' added
- FIXME added in 'gcpukernel.hpp'

* INF_ENGINE fix
- preprocessing clauses added not to run the sample without Inference Engine

* INF_ENGINE fix 2
- warnings removed

* Fixes
- checking IE version cut as there is no dependency
- some alignments fixed
- the comment about preprocessing commands fixed

* ie::backend() issue fix (according to dmatveev)
- as the sample needs the cv::gapi::ie::backend() to be defined regardless of having IE or not, there is its throw-error definition in `giebackend.cpp` now (by dmatveev)
- for the same reason, #includes in `giebackend.hpp` are fixed
- HAVE_INF_ENGINE check is removed from the sample

modules/gapi/src/backends/ie/giebackend.cpp

@@ -6,6 +6,10 @@
 
 #include "precomp.hpp"
 
+// needs to be included regardless if IE is present or not
+// (cv::gapi::ie::backend() is still there and is defined always)
+#include "backends/ie/giebackend.hpp"
+
 #ifdef HAVE_INF_ENGINE
 
 #if INF_ENGINE_RELEASE <= 2018050000
@@ -32,7 +36,6 @@
 #include "compiler/gobjref.hpp"
 #include "compiler/gmodel.hpp"
 
-#include "backends/ie/giebackend.hpp"
 #include "backends/ie/util.hpp"
 
 #include "api/gbackend_priv.hpp" // FIXME: Make it part of Backend SDK!
@@ -601,4 +604,9 @@ InferenceEngine::Blob::Ptr cv::gapi::ie::util::to_ie(cv::Mat &blob) {
     return wrapIE(blob);
 }
 
+#else
+cv::gapi::GBackend cv::gapi::ie::backend() {
+    // Still provide this symbol to avoid linking issues
+    util::throw_error(std::runtime_error("G-API has been compiled without OpenVINO IE support"));
+}
 #endif // HAVE_INF_ENGINE

modules/gapi/src/backends/ie/giebackend.hpp

@@ -7,6 +7,9 @@
 #ifndef OPENCV_GAPI_GIEBACKEND_HPP
 #define OPENCV_GAPI_GIEBACKEND_HPP
 
+// Include anyway - cv::gapi::ie::backend() still needs to be defined
+#include "opencv2/gapi/infer/ie.hpp"
+
 #ifdef HAVE_INF_ENGINE
 
 #include <ade/util/algorithm.hpp> // type_list_index
@@ -35,7 +38,6 @@
 
 #include <opencv2/gapi/garg.hpp>
 #include <opencv2/gapi/gproto.hpp>
-#include <opencv2/gapi/infer/ie.hpp>
 
 #include "api/gorigin.hpp"
 #include "backends/common/gbackend.hpp"