Use SplitInternalLoops before Polygon is creation (Issue #881)

[DmytroMuravskyi]

BACKGROUND:

• This is fix for issue Profile.Difference() Exception 'Unable to clean up bad polygon resulting from a polygon boolean operation' #881
• Clipper lib can produce single loop that in fact contains more than one Polygon.
• There is existing SplitInternalLoops function but often I has no opportunity to be called since exception is thrown.
• I only covered Profile creation but issue is reproducible for both Polygon and Profile.

DESCRIPTION:

• Internal SplitInternalLoops and RemoveSequentialDuplicates are moved to Vector3 static section for better reusability.
• In ToPolygon, use DeleteVerticesForOverlappingEdges, RemoveSequentialDuplicates and SplitInternalLoops so Polygons are created from already split loops.
• Internal ToPolygon now returns list of Polygon.
• Change Profile.Union so it uses PolyTree as other functions and not List<List<ClipperLib.IntPoint>>();
• Remove internal Profile.Clip as it was not used and was hard to update. In my opinion there is no value in keeping it around.

TESTING:

• Added DifferenceProduceTwoResultsInOneLoop() test.

REQUIRED:

• All changes are up to date in CHANGELOG.md.

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