Repository: opencv
Issue: Test failures  on opencv_test_gapi MulTestGPU/MathOpTest.MatricesAccuracyTest/
Labels: category: core,platform: arm,category: g-api / gapi

##### System information (version)
- OpenCV => 4.5.2
- Operating System / Platform => Linux aarch64 / i.mx 8M board
- Compiler => gcc
##### Detailed description
[ FAILED ] , where GetParam() = (8UC3, 1280x720, SAME_TYPE, 0xaaaac93df890, MUL, false, 0.5, false)
[ FAILED ] MulTestGPU/MathOpTest.MatricesAccuracyTest/79, where GetParam() = (8UC3, 1280x720, 8UC1, 0xaaaac93df890, MUL, false, 0.5, false)
[ FAILED ] MulTestGPU/MathOpTest.MatricesAccuracyTest/97, where GetParam() = (8UC3, 640x480, SAME_TYPE, 0xaaaac93df890, MUL, false, 0.5, false)
[ FAILED ] MulTestGPU/MathOpTest.MatricesAccuracyTest/103, where GetParam() = (8UC3, 640x480, 8UC1, 0xaaaac93df890, MUL, false, 0.5, false)
[ FAILED ] MulTestGPU/MathOpTest.MatricesAccuracyTest/121, where GetParam() = (8UC3, 128x128, SAME_TYPE, 0xaaaac93df890, MUL, false, 0.5, false)
[ FAILED ] MulTestGPU/MathOpTest.MatricesAccuracyTest/127, where GetParam() = (8UC3, 128x128, 8UC1, 0xaaaac93df890, MUL, false, 0.5, false)

These issues are occured on gapi with OpenCL. I found these issues are caused by difference of round method between ocv and ocl.
- ocv always round down
- gapi use ocl method convert_uchar4_sat_rte. It will round down to nearest even and round up to nearest value.

I don't know if it's real a problem, the round to even method used by ocl seems more scientific, it can reduce systematic bias. 

Please let me know how you will take care of these failures, Thankyou.

##### Steps to reproduce

bin/opencv_test_gapi --gtest_filter=MulTestGPU/MathOpTest.MatricesAccuracyTest/73
[MulTestGPU_MathOpTest_MatricesAccuracyTest_73_data.zip](https://github.com/opencv/opencv/files/6820716/MulTestGPU_MathOpTest_MatricesAccuracyTest_73_data.zip)


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