GAPI (fluid): optimization of Separable filter (#13221)

* GAPI (fluid): Separable filter: performance test

* GAPI (fluid): enable all performance tests

* GAPI: separable filters: alternative code for Sobel

* GAPI (fluid): hide unused old code for Sobel filter

* GAPI (fluid): especial code for Sobel if U8 into S16

* GAPI (fluid): back to old code for Sobel

* GAPI (fluid): run_sepfilter3x3_impl() with CPU dispatcher

* GAPI (fluid): run_sepfilter3x3_impl(): fix compiler warnings

* GAPI (fluid): new engine for separable filters (but Sobel)

* GAPI (fluid): new performance engine for Sobel

* GAPI (fluid): Sepfilters performance: fixed compilation error