ximgproc: tolerate the rounding error

[tomoaki0705]

• edge drawing is NOT bit-exact

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On some platforms such as Jetson Nano or Rapsberry Pi 3/4, this test ximgproc_ED.ManySmallCircles fails with following message

[ RUN      ] ximgproc_ED.ManySmallCircles
/opencv_contrib/modules/ximgproc/test/test_fld.cpp:295: Failure
Expected equality of these values:
  lines.size()
    Which is: 6266
  lines_size
    Which is: 6264
[  FAILED  ] ximgproc_ED.ManySmallCircles (486 ms)

This is caused due to the rounding error

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• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake