update gpu tests

fixed BFM warning ("cannot tell what pointer points to")

minor fixes and refactoring of GPU module

added assertion after all kernels calls

module reorganization: added folder with pure device functions, cuda_shared.hpp renamed to internal_shared.hpp

refactored gpu module.

added supports of 4-channels image to gpu::minMax and gpu::cvtColor for RGB <-> YCrCB, RGB <-> YUV and RGB <-> XYZ color conversion.

added RGB <-> XYZ color conversion.
gpu morphology minor fix.

added gpu::cvtColor for BGR5x5 <-> BGR and BGR5x5 <-> Gray.

minor refactoring of GPU module and GPU tests, split arithm and imgproc parts.