[G-API]: Fluid: add mask, extend convertTo for CV_16S

* Add Fluid `mask` kernel + Acc. and Perf. tests
 - simple cycle implementation with restrictions on mask type and input/output type like in OCV/GPU kernels (mask - CV_8UC1 only, input/output - CV_8UC1, CV_16UC1, CV_16SC1)

* Added convertions from/to 16S

* `convertTo()` perf tests refactoring
 - add testing of `alpha` and `beta` parameters
 - fixed unreliable comparison
 - added instances to OCV, Fluid and GPU tests according to the changes

* Addressing comments
 - fixed multiple-channel mistake - prohibited multiple-channeling

* Reduced perf tests