Merge pull request #13290 from elatkin:el/gapi_perf_filter2d

Merge pull request #13290 from elatkin:el/gapi_perf_filter2d

GAPI (fluid): Filter 2D optimization (#13290)

* GAPI (fluid): Filter 2D optimization: speedup 13x if float, 2x if integral

* GAPI (fluid): Filter 2D speedup 8x if output is short/ushort

* GAPI (fluid): Filter 2D speedup 7x if output is uchar

* GAPI (fluid): Filter 2D optimization: fixed compiler warnings

* GAPI (fluid): fix compiler warnings on Mac

* GAPI (fluid): fix compiler warnings on Mac

* GAPI (fluid): fix compiler errors on VS2015

* GAPI (fluid): fix compiler errors on VS2015

* GAPI (fluid): fix compiler errors on VS2015