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

* GAPI: Sobel 3x3 with FP32 input

* GAPI: Sobel 3x3 with FP32 input, v2

G-API: Introduce new `reshape()` API (#12990)

* Moved initFluidUnits, initLineConsumption, calcLatency, calcSkew to separate functions

* Added Fluid::View::allocate method (moved allocation logic from constructor)

* Changed util::zip to util::indexed, utilized collectInputMeta in GFluidExecutable constructor

* Added makeReshape method to FluidExecutable

* Removed m_outputRoi from GFluidExecutable

* Added reshape feature

* Added switch of resize mapper if agent ratio was changed

* Added more TODOs and renamed a function

* G-API reshape(): add missing `override` specifiers

Fix warnings on all platforms

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

* added threshold to g-api imgproc tests

* trailing spaces remove

* try to fix warnings

* fix license header

* fix random numbers issue

* fix Sobel test

* license headers + precompiled headers included

* Renamed Sobel operator GAPI kernel to match with OpenCV naming rules

* Fixed perf tests

* Small refactoring to check CI issue

* Refactored alignment for kernel wrappers in imgproc.hpp

* G-API Fluid basic heterogeneity support: initial upload

* G-API Fluid heterogeneity: address some coding style issues

* G-API Fluid heterogeneity: fix compiler warnings

* G-API Fluid heterogeneity: fix warnings on Windows & ARMv7

* G-API Fluid heterogeneity: finally fix Windows warnings

* G-API Fluid heterogeneity: fix dangling reference problem

Several Resize optimizations count on fetching multiple input lines at
once to do interpolation more efficiently.

At the moment, Fluid backend supports only LPI=1 for Resize kernels.

This patch introduces scheduling support for Resizes with LPI>1 and
covers these cases with new tests.

The support is initially written by Ruslan Garnov.

* Update G-API code base to 27-Sep-18

Changes mostly improve standalone build support

* G-API code base update 28-09-2018

* Windows/Documentation warnings should be fixed
* Fixed stability issues in Fluid backend
* Fixed precompiled headers issues in G-API source files

* G-API code base update 28-09-18 EOD

* Fixed several static analysis issues
* Fixed issues found when G-API is built in a standalone mode

* 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