Merge pull request #18261 from OrestChura:oc/fluid_convert_mask
[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