Merge pull request #15907 from rgarnov:rg/unique_ptr_for_priv_in_fluid

diff --cc modules/gapi/include/opencv2/gapi/fluid/gfluidbuffer.hpp
Simple merge

diff --cc modules/gapi/include/opencv2/gapi/fluid/gfluidkernel.hpp
Simple merge

diff --cc modules/gapi/src/backends/fluid/gfluidbackend.cpp
Simple merge

diff --cc modules/gapi/src/backends/fluid/gfluidbackend.hpp
Simple merge

diff --cc modules/gapi/src/backends/fluid/gfluidbuffer.cpp
Simple merge

diff --cc modules/gapi/src/backends/fluid/gfluidbuffer_priv.hpp
Simple merge

diff --cc modules/gapi/test/gapi_kernel_tests.cpp
Simple merge