Fluid Internal Parallelism

- Added new graph compile time argument to specify multiple independent
ROIs (Tiles)
 - Added new "executable" with serial loop other user specified
ROIs(Tiles)
 - refactored graph traversal code into separate function to be called
once
 - added saturate cast to Fluid AddCsimple test kernel

modules/gapi/include/opencv2/gapi/fluid/gfluidkernel.hpp

@@ -99,12 +99,22 @@ struct GFluidOutputRois
     std::vector<cv::gapi::own::Rect> rois;
 };
 
+struct GFluidParallelOutputRois
+{
+    std::vector<GFluidOutputRois> parallel_rois;
+};
+
 namespace detail
 {
 template<> struct CompileArgTag<GFluidOutputRois>
 {
     static const char* tag() { return "gapi.fluid.outputRois"; }
 };
+template<> struct CompileArgTag<GFluidParallelOutputRois>
+{
+    static const char* tag() { return "gapi.fluid.parallelOutputRois"; }
+};
+
 } // namespace detail
 
 namespace detail