Added to CPU path only.
OpenCL code path still expects NV12 only (according to Intel OpenCL extension)

Do not try to process empty inputs.

Details 12176

remove from DNN module headers

	* Keep image aspect ratio in resize called in grad_system and
weak_order
	* Bug correction in loop inside Decolor::gradvector

* core(test): merge hang test

* core(merge/split): fix intrin optimization

fixed call open(int id) failed

All vx_call() must be wrapped into own simd128/simd256/simd512 namespace

```
namespace CV__SIMD_NAMESPACE {
... vx_call declaration is here ...
}
```

- use '3.4.x' cache name for current maintenance series (there are no serious changes between releases)
- message is shown only once during creation of new cache directory
- use OPENCV_CACHE_SHOW_CLEANUP_MESSAGE=0 to hide this warning

imgproc: update cornerSubPix documentation