OCL, IPP: Small fixes

- Fixed compilation warning in perf_dxt.cpp.
- Increased epsilon for Core_Eigen test.
- Added ROUNDING_EPSILON to calc_back_project.cl for identical rounding on all platforms (issue found on Nvidia GTX650 with test_loop_times=30).
- Disabled `ippiFilterBoxBorder_*` in case maskSize equal roiSize for any dimension since it returns `ippStsMaskSizeErr`.
- Set minimum matrix size for AmdBlas::gemm to 20 since it works incorrect for small sizes (found with test_loop_times=30).

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