G-API: Implement color-convert kernels (#14513)

* Implement color-convert kernels

* Fix rgb2yuv422 reference version

* Fix comments to review

* Restore NV12toBGR in imgproc.hpp

* Add accuracy tests

* Fix doxygen

* Fix ref version yuv422

* Fix warnings

* Fix typos

* Fix simd version yuv422

* Fix warnings

* Fix compile error

* Fix warning

* Remove comment