G-API GPU-OpenCL backend (#13008)
* gpu/ocl backend core
* accuracy tests added and adjusted + license headers
* GPU perf. tests added; almost all adjusted to pass
* all tests adjusted and passed - ready for pull request
* missing license headers
* fix warning (workaround RGB2Gray)
* fix c++ magic
* precompiled header
* white spaces
* try to fix warning and blur test
* try to fix Blur perf tests
* more alignments with the latest cpu backend
* more gapi tests refactoring + 1 more UB issue fix + more informative tolerance exceed reports
* white space fix
* try workaround for SumTest
* GAPI_EXPORTS instead CV_EXPORTS
Made scale parameter optional for mul kernel wrapper (#12949)
* Added missed operator*(GMat, GMat). Made scale parameter optional for mul kernel.
* Fixed perf test for mul(GMat, GMat) kernel
* Removed operator*(GMat, GMat) as not needed
* G-API Initial code upload
* Update G-API code base to Sep-24-2018
* The majority of OpenCV buildbot problems was addressed
* Update G-API code base to 24-Sep-18 EOD
* G-API code base update 25-Sep-2018
* Linux warnings should be resolved
* Documentation build should become green
* Number of Windows warnings should be reduced
* Update G-API code base to 25-Sep-18 EOD
* ARMv7 build issue should be resolved
* ADE is bumped to latest version and should fix Clang builds for macOS/iOS
* Remaining Windows warnings should be resolved
* New Linux32 / ARMv7 warnings should be resolved
* G-API code base update 25-Sep-2018-EOD2
* Final Windows warnings should be resolved now
* G-API code base update 26-Sep-2018
* Fixed issues with precompiled headers in module and its tests
