ximgproc: fix angle difference normalization in FastLineDetector merging

[AYELET0527661122]

Fixes #3956
Fix angle difference computation in FastLineDetector::mergeSegments.

Previously the angular difference was computed using a direct absolute
difference between segment angles. As a result, collinear segments with
opposite directions (~pi radians apart) could fail to merge.

The angle difference is now normalized so that segments differing by pi
radians are treated as equivalent.

A regression test (mergeOppositeDirections) was added to verify that
touching parallel segments are merged correctly for both same and
opposite directions.

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• There is a reference to the 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

[AlexanderHaritonov]

Please remove trailing whitespaces.
See https://pullrequest.opencv.org/buildbot/builders/precommit-contrib_docs/builds/101694/steps/whitespace%20opencv_contrib/logs/stdio